DOCSLIB.ORG

Constant Current Source: Active Current Source Applications Simple

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Constant Current Source: Active Current Source Applications Simple Constant Current Source: A Current source is an electrical or electronic device that delivers or absorbs electric current. A current source is the dual of a voltage source. Active constant current sources are often used in circuits. Often these constant current sources use transistors, although FETs and where applicable, vacuum tunes of thermionic valves can be used as well. No real current source is ideal (no unlimited energy sources exist) and all have a finite internal resistance (none can supply unlimited voltage). However, the internal resistance of a physical current source is effectively modeled in circuit analysis by combining a non zero resistance in parallel with an ideal current source (The Norton equivalent circuit) It is possible to make an active constant current source using a single transistor, although more comprehensive designs are also available using additional components. Active current source applications Current sources are needed in a number of different areas of electronics circuit design. Current sources can be used to bias transistors and can also be used as active loads for high gain amplifier stages. They may also be used as the emitter sources for differential amplifiers - for example they may be used in the transistor long tailed pair. They may also be used as wide voltage range pull-up links within power supplies and other wide voltage range circuits. If ordinary resistors were used then the current would vary considerably over the voltage range. One common example of the use of current sources is to drive a Zener diode in a regulator circuit. Keeping the current constant regardless of the current taken by the series pass transistor in the circuit helps maintain a much better level of regulation. Also stand-alone current sources are also needed in processes ranging including electrochemistry and electrophoresis. Simple resistor current source circuit The simplest form of constant current circuit is a simple resistor. If the voltage of the source voltage is much higher than the voltage where the current is required, then the output current will be almost independent of the load. Under these circumstances the current can be calculated very easily as it is approximately I = V / R because Vload (the voltage across the load) is much smaller than V (the voltage of the source). This simple form of current source has many limitations: The high values of resistance needed dissipate power making circuits inefficient. High source voltages are needed and are not always easily available. Variations in load may cause some current variations if sufficiently high values of source voltage are not available. Transistor active constant current source basics The simple use of a transistor enables a far more effective current source to be made. The current source operates because of the fact that the collector current in a transistor circuit is Β times the base current. This is independent of the collector voltage, provided that there is sufficient voltage to drive the current through the load device in the collector. In this circuit, the collector current is β times the base current. Normally β is large and therefore it can be assumed that the emitter current which is (β + 1) times the base current and the collector current which is β times the base current are the same. In view of this it is a simple matter to design the circuit for a given current. Ie = (β + 1) Ib Iload =Ic =βIb Iload = β Ve(β + 1) Re Iload =Vb − 0.6Re NB: This assumes the use of a silicon transistor as the base Single Transistor Active emitter drop is given as 0.6V Current Source By setting the resistors R1 and R2 it is possible to set the base voltage. The emitter voltage will be 0.6 volts less, assuming a silicon transistor. By knowing the emitter voltage, it is possible to calculate the emitter current from a simple knowledge of Ohms law. Simple stabilized active current source circuit In order to remove any fluctuations in current arising from changes in supply voltage it is a simple matter to add some regulation to the basic circuit. This is achieved by replacing R2 with a Zener or voltage reference diode. The same equations apply as before, but the only difference is that the base voltage is held at a more constant level as a result of the presence of the Zener, voltage reference diode Transistor active current source using Zener diode to improve stability Active current source temperature dependence One of the main disadvantages of the basic active current source is that it is dependent upon temperature to a degree. For many applications this may not be important, but where very tightly controlled conditions are needed, the temperature performance may be very important. There are two main variations that occur: Variations of Vbe with respect to temperature The effects of the change in Vbe caused by temperature are approximately -2mV/°C. This results in a variation of Vce. It is possible to calculate an approximate relationship: ΔVbe approximately equals -0.0001ΔVce. This can be minimized by choosing an emitter resistor value sufficiently large to ensure that emitter voltage changes of tens of mille volts will only be a small proportion of the overall emitter voltage. However care must be taken to ensure that there is still sufficient remaining voltage between the collector and the rail to drive the current through the load and take up any variations in supply voltage. Variations of β with respect to temperature This may not be a major issue and any variations can be minimized by choosing a transistor with a high value of Β / Hfe. In this way the base current contribution to the emitter current is minimized and the variations reduced as far as possible. Active current source circuits with good temperature stability It is possible to design transistor active current source circuits where the inherent temperature stability is better than the simple circuits given above. One of the simplest circuits is to employ one that uses both NPN and PNP transistors. In the circuit shown, the Vbe voltage drop changes in TR1 are compensated by those in TR2. It should be noted in this circuit that R3 is a pull up resistor for the collector of TR1 because the base of TR2 can sink current but not source it. The circuits above all include transistors, but other devices including FETs and even vacuum tubes / thermionic valves can also be used. When using other devices as the active device in a current source, the biasing arrangements and circuit need to account for the fact that both FETs and valves / tubes are voltage driven rather than current driven. Nevertheless they can still be used just as effectively. Transistor active current sources are used in many areas, particularly within integrated circuits and some battery chargers. They enable a fixed or controlled current to be Temperature compensated supplied independent of the voltage (within limits) and as such transistor active current source they are very useful. Aim To construct a zener diode voltage regulator and measure its line and load regulation. Apparatus Zener diode, resistor, variable DC power supply, milliammeter, voltmeter, Rheostat and wire. Theory The Zener diode is like a general-purpose signal diode. When biased in the forward direction it behaves just like a normal signal diode, but when a reverse voltage is applied to it, the voltage remains constant for a wide range of currents. Avalanche Breakdown: There is a limit for the reverse voltage. Reverse voltage can increase until the diode breakdown voltage reaches. This point is called Avalanche Breakdown region. At this stage maximum current will flow through the zener diode. This breakdown point is referred as “Zener voltage”. Fig 1: Zener diode The Zener Diode is used in its "reverse bias". From the I-V Characteristics curve we can study that the zener diode has a region in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the diode and remains nearly constant even with large changes in current as long as the zener diodes current remains between the breakdown current IZ(min) and the maximum current rating IZ(max). This ability to control itself can be used to great effect to regulate or stabilise a voltage source against supply or load variations. The fact that the voltage across the diode in the breakdown region is almost constant turns out to be an important application of the zener diode as a voltage regulator Characteristics Figure 2 shows the current versus voltage curve for a Zener diode. Observe the nearly constant voltage in the breakdown region. Fig 2: Zener diode characteristic curve The forward bias region of a Zener diode is identical to that of a regular diode. The typical forward voltage at room temperature with a current of around 1 mA is around 0.6 volts. In the reverse bias condition the Zener diode is an open circuit and only a small leakage current is flowing as shown on the exaggerated plot. As the breakdown voltage is approached the current will begin to avalanche. The initial transition from leakage to breakdown is soft but then the current rapidly increases as shown on the plot. The voltage across the Zener diode in the breakdown region is very nearly constant with only a small increase in voltage with increasing current. At some high current level the power dissipation of the diode becomes excessive and the part is destroyed. There is a minimum Zener current, Iz(min), that places the operating point in the desired breakdown. There is a maximum Zener current, Iz(max), at which the power dissipation drives the junction temperature to the maximum allowed.
Load more

Recommended publications
  • Fundamentals of Microelectronics Chapter 3 Diode Circuits
    9/17/2010 Fundamentals of Microelectronics CH1 Why Microelectronics? CH2 Basic Physics of Semiconductors CH3 Diode Circuits CH4 Physics of Bipolar Transistors CH5 Bipolar Amplifiers CH6 Physics of MOS Transistors CH7 CMOS Amplifiers CH8 Operational Amplifier As A Black Box 1 Chapter 3 Diode Circuits 3.1 Ideal Diode 3.2 PN Junction as a Diode 3.3 Applications of Diodes 2 1 9/17/2010 Diode Circuits After we have studied in detail the physics of a diode, it is time to study its behavior as a circuit element and its many applications. CH3 Diode Circuits 3 Diode’s Application: Cell Phone Charger An important application of diode is chargers. Diode acts as the black box (after transformer) that passes only the positive half of the stepped-down sinusoid. CH3 Diode Circuits 4 2 9/17/2010 Diode’s Action in The Black Box (Ideal Diode) The diode behaves as a short circuit during the positive half cycle (voltage across it tends to exceed zero), and an open circuit during the negative half cycle (voltage across it is less than zero). CH3 Diode Circuits 5 Ideal Diode In an ideal diode, if the voltage across it tends to exceed zero, current flows. It is analogous to a water pipe that allows water to flow in only one direction. CH3 Diode Circuits 6 3 9/17/2010 Diodes in Series Diodes cannot be connected in series randomly. For the circuits above, only a) can conduct current from A to C. CH3 Diode Circuits 7 IV Characteristics of an Ideal Diode V V R = 0⇒ I = = ∞ R = ∞⇒ I = = 0 R R If the voltage across anode and cathode is greater than zero, the resistance of an ideal diode is zero and current becomes infinite.
    [Show full text]
  • Chapter 2 Basic Concepts in RF Design
    Chapter 2 Basic Concepts in RF Design 1 Sections to be covered • 2.1 General Considerations • 2.2 Effects of Nonlinearity • 2.3 Noise • 2.4 Sensitivity and Dynamic Range • 2.5 Passive Impedance Transformation 2 Chapter Outline Nonlinearity Noise Impedance Harmonic Distortion Transformation Compression Noise Spectrum Intermodulation Device Noise Series-Parallel Noise in Circuits Conversion Matching Networks 3 The Big Picture: Generic RF Transceiver Overall transceiver Signals are upconverted/downconverted at TX/RX, by an oscillator controlled by a Frequency Synthesizer. 4 General Considerations: Units in RF Design Voltage gain: rms value Power gain: These two quantities are equal (in dB) only if the input and output impedance are equal. Example： an amplifier having an input resistance of R0 (e.g., 50 Ω) and driving a load resistance of R0 : 5 where Vout and Vin are rms value. General Considerations: Units in RF Design “dBm” The absolute signal levels are often expressed in dBm (not in watts or volts); Used for power quantities, the unit dBm refers to “dB’s above 1mW”. To express the signal power, Psig, in dBm, we write 6 Example of Units in RF An amplifier senses a sinusoidal signal and delivers a power of 0 dBm to a load resistance of 50 Ω. Determine the peak-to-peak voltage swing across the load. Solution: a sinusoid signal having a peak-to-peak amplitude of Vpp an rms value of Vpp/(2√2), 0dBm is equivalent to 1mW, where RL= 50 Ω thus, 7 Example of Units in RF A GSM receiver senses a narrowband (modulated) signal having a level of -100 dBm.
    [Show full text]
  • The Basics of Power the Background of Some of the Electronics
    We Often talk abOut systeMs from a “in front of the (working) screen” or a Rudi van Drunen “software” perspective. Behind all this there is a complex hardware architecture that makes things work. This is your machine: the machine room, the network, and all. Everything has to do with electronics and electrical signals. In this article I will discuss the basics of power the background of some of the electronics, Rudi van Drunen is a senior UNIX systems consul- introducing the basics of power and how tant with Competa IT B.V. in the Netherlands. He to work with it, so that you will be able to also has his own consulting company, Xlexit Tech- nology, doing low-level hardware-oriented jobs. understand the issues and calculations that [email protected] are the basis of delivering the electrical power that makes your system work. There are some basic things that drive the electrons through your machine. I will be explaining Ohm’s law, the power law, and some aspects that will show you how to lay out your power grid. power Law Any piece of equipment connected to a power source will cause a current to flow. The current will then have the device perform its actions (and produce heat). To calculate the current that will be flowing through the machine (or light bulb) we divide the power rating (in watts) by the voltage (in volts) to which the system is connected. An ex- ample here is if you take a 100-watt light bulb and connect this light bulb to the wall power voltage of 115 volts, the resulting current will be 100/115 = 0.87 amperes.
    [Show full text]
  • VOLTAGE REGULATORS 1. Zener Controlled Transistor Voltage Regulator
    VOLTAGE REGULATORS A voltage regulator is a voltage stabilizer that is designed to automatically stabilize a constant voltage level. A voltage regulator circuit is also used to change or stabilize the voltage level according to the necessity of the circuit. Thus, a voltage regulator is used for two reasons:- 1. To regulate or vary the output voltage of the circuit. 2. To keep the output voltage constant at the desired value in-spite of variations in the supply voltage or in the load current. To know more on the basics of this subject, you may also refer Regulated Power Supply. Voltage regulators find their applications in computers, alternators, power generator plants where the circuit is used to control the output of the plant. Voltage regulators may be classified as electromechanical or electronic. It can also be classified as AC regulators or DC regulators. We have already explained about IC Voltage Regulators. Electronic Voltage Regulator All electronic voltage regulators will have a stable voltage reference source which is provided by the reverse breakdown voltage operating diode called zener diode. The main reason to use a voltage regulator is to maintain a constant dc output voltage. It also blocks the ac ripple voltage that cannot be blocked by the filter. A good voltage regulator may also include additional circuits for protection like short circuits, current limiting circuit, thermal shutdown, and over voltage protection. Electronic voltage regulators are designed by any of the three or a combination of any of the three regulators given below. 1. Zener Controlled Transistor Voltage Regulator A zener controlled voltage regulator is used when the efficiency of a regulated power supply becomes very low due to high current.
    [Show full text]
  • ON Semiconductor Is
    ON Semiconductor Is Now To learn more about onsemi™, please visit our website at www.onsemi.com onsemi and and other names, marks, and brands are registered and/or common law trademarks of Semiconductor Components Industries, LLC dba “onsemi” or its affiliates and/or subsidiaries in the United States and/or other countries. onsemi owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of onsemi product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent-Marking.pdf. onsemi reserves the right to make changes at any time to any products or information herein, without notice. The information herein is provided “as-is” and onsemi makes no warranty, representation or guarantee regarding the accuracy of the information, product features, availability, functionality, or suitability of its products for any particular purpose, nor does onsemi assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. Buyer is responsible for its products and applications using onsemi products, including compliance with all laws, regulations and safety requirements or standards, regardless of any support or applications information provided by onsemi. “Typical” parameters which may be provided in onsemi data sheets and/ or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. onsemi does not convey any license under any of its intellectual property rights nor the rights of others.
    [Show full text]
  • Special Diodes 2113
    CHAPTER54 Learning Objectives ➣ Zener Diode SPECIAL ➣ Voltage Regulation ➣ Zener Diode as Peak Clipper DIODES ➣ Meter Protection ➣ Zener Diode as a Reference Element ➣ Tunneling Effect ➣ Tunnel Diode ➣ Tunnel Diode Oscillator ➣ Varactor Diode ➣ PIN Diode ➣ Schottky Diode ➣ Step Recovery Diode ➣ Gunn Diode ➣ IMPATT Diode Ç A major application for zener diodes is voltage regulation in dc power supplies. Zener diode maintains a nearly constant dc voltage under the proper operating conditions. 2112 Electrical Technology 54.1. Zener Diode It is a reverse-biased heavily-doped silicon (or germanium) P-N junction diode which is oper- ated in the breakdown region where current is limited by both external resistance and power dissipa- tion of the diode. Silicon is perferred to Ge because of its higher temperature and current capability. As seen from Art. 52.3, when a diode breaks down, both Zener and avalanche effects are present although usually one or the other predominates depending on the value of reverse voltage. At reverse voltages less than 6 V, Zener effect predominates whereas above 6 V, avalanche effect is predomi- nant. Strictly speaking, the first one should be called Zener diode and the second one as avalanche diode but the general practice is to call both types as Zener diodes. Zener breakdown occurs due to breaking of covalent bonds by the strong electric field set up in the depletion region by the reverse voltage. It produces an extremely large number of electrons and holes which constitute the reverse saturation current (now called Zener current, Iz) whose value is limited only by the external resistance in the circuit.
    [Show full text]
  • Power Supply Using Power Transistors
    Section F6: Power Supply Using Power Transistors In this section of our studies we will be looking at the design of power supplies using power transistors. We discussed the concepts of rectification and filtering using regular and zener diodes in Section B, and we are going to start this section with a twist on our previous work – adding a BJT. After this introduction using discrete transistors, we will be examining design approaches using integrated circuits – both the 7800 series of integrated circuit regulators and the LM317 adjustable regulator. Power Supply Using Discrete Components In Section B9 (3.4 of your text), we used the zener diode as the voltage-controlling device in the design of a regulated power supply. The figure to the right is a modified version of Figure 8.20 in your text, where the notation has been changed to correctly reproduce Figure 3.39. As we saw earlier, this is a fairly well behaved circuit that supplies a nearly constant output voltage over a wide range of currents. However, as we always must, we can do better. A better regulation may be obtained if the zener diode is connected to the base circuit of a power transistor in the EF (CC) configuration as shown below (Figure 8.21 of your text). In the configuration above, the transistor is referred to as a pass transistor. Because of the current amplifying properties of the BJT in the EF (CC)configuration, the current through the zener diode may be small. With the smaller current, there is little voltage drop across the diode resistance, which allows the zener to approximate an ideal constant voltage source.
    [Show full text]
  • Notes for Lab 1 (Bipolar (Junction) Transistor Lab)
    ECE 327: Electronic Devices and Circuits Laboratory I Notes for Lab 1 (Bipolar (Junction) Transistor Lab) 1. Introduce bipolar junction transistors • “Transistor man” (from The Art of Electronics (2nd edition) by Horowitz and Hill) – Transistors are not “switches” – Base–emitter diode current sets collector–emitter resistance – Transistors are “dynamic resistors” (i.e., “transfer resistor”) – Act like closed switch in “saturation” mode – Act like open switch in “cutoff” mode – Act like current amplifier in “active” mode • Active-mode BJT model – Collector resistance is dynamically set so that collector current is β times base current – β is assumed to be very high (β ≈ 100–200 in this laboratory) – Under most conditions, base current is negligible, so collector and emitter current are equal – β ≈ hfe ≈ hFE – Good designs only depend on β being large – The active-mode model: ∗ Assumptions: · Must have vEC > 0.2 V (otherwise, in saturation) · Must have very low input impedance compared to βRE ∗ Consequences: · iB ≈ 0 · vE = vB ± 0.7 V · iC ≈ iE – Typically, use base and emitter voltages to find emitter current. Finish analysis by setting collector current equal to emitter current. • Symbols – Arrow represents base–emitter diode (i.e., emitter always has arrow) – npn transistor: Base–emitter diode is “not pointing in” – pnp transistor: Emitter–base diode “points in proudly” – See part pin-outs for easy wiring key • “Common” configurations: hold one terminal constant, vary a second, and use the third as output – common-collector ties collector
    [Show full text]
  • I. Common Base / Common Gate Amplifiers
    I. Common Base / Common Gate Amplifiers - Current Buffer A. Introduction • A current buffer takes the input current which may have a relatively small Norton resistance and replicates it at the output port, which has a high output resistance • Input signal is applied to the emitter, output is taken from the collector • Current gain is about unity • Input resistance is low • Output resistance is high. V+ V+ i SUP ISUP iOUT IOUT RL R is S IBIAS IBIAS V− V− (a) (b) B. Biasing = /α ≈ • IBIAS ISUP ISUP EECS 6.012 Spring 1998 Lecture 19 II. Small Signal Two Port Parameters A. Common Base Current Gain Ai • Small-signal circuit; apply test current and measure the short circuit output current ib iout + = β v r gmv oib r − o ve roc it • Analysis -- see Chapter 8, pp. 507-509. • Result: –β ---------------o ≅ Ai = β – 1 1 + o • Intuition: iout = ic = (- ie- ib ) = -it - ib and ib is small EECS 6.012 Spring 1998 Lecture 19 B. Common Base Input Resistance Ri • Apply test current, with load resistor RL present at the output + v r gmv r − o roc RL + vt i − t • See pages 509-510 and note that the transconductance generator dominates which yields 1 Ri = ------ gm µ • A typical transconductance is around 4 mS, with IC = 100 A • Typical input resistance is 250 Ω -- very small, as desired for a current amplifier • Ri can be designed arbitrarily small, at the price of current (power dissipation) EECS 6.012 Spring 1998 Lecture 19 C. Common-Base Output Resistance Ro • Apply test current with source resistance of input current source in place • Note roc as is in parallel with rest of circuit g v m ro + vt it r − oc − v r RS + • Analysis is on pp.
    [Show full text]
  • Precision Current Sources and Sinks Using Voltage References
    Application Report SNOAA46–June 2020 Precision Current Sources and Sinks Using Voltage References Marcoo Zamora ABSTRACT Current sources and sinks are common circuits for many applications such as LED drivers and sensor biasing. Popular current references like the LM134 and REF200 are designed to make this choice easier by requiring minimal external components to cover a broad range of applications. However, sometimes the requirements of the project may demand a little more than what these devices can provide or set constraints that make them inconvenient to implement. For these cases, with a voltage reference like the TL431 and a few external components, one can create a simple current bias with high performance that is flexible to fit meet the application requirements. Current sources and sinks have been covered extensively in other Texas Instruments application notes such as SBOA046 and SLYC147, but this application note will cover other common current sources that haven't been previously discussed. Contents 1 Precision Voltage References.............................................................................................. 1 2 Current Sink with Voltage References .................................................................................... 2 3 Current Source with Voltage References................................................................................. 4 4 References ................................................................................................................... 6 List of Figures 1 Current
    [Show full text]
  • Lecture 12 Digital Circuits (II) MOS INVERTER CIRCUITS
    Lecture 12 Digital Circuits (II) MOS INVERTER CIRCUITS Outline • NMOS inverter with resistor pull-up –The inverter • NMOS inverter with current-source pull-up • Complementary MOS (CMOS) inverter • Static analysis of CMOS inverter Reading Assignment: Howe and Sodini; Chapter 5, Section 5.4 6.012 Spring 2007 Lecture 12 1 1. NMOS inverter with resistor pull-up: Dynamics •CL pull-down limited by current through transistor – [shall study this issue in detail with CMOS] •CL pull-up limited by resistor (tPLH ≈ RCL) • Pull-up slowest VDD VDD R R VOUT: VOUT: HI LO LO HI V : VIN: IN C LO HI CL HI LO L pull-down pull-up 6.012 Spring 2007 Lecture 12 2 1. NMOS inverter with resistor pull-up: Inverter design issues Noise margins ↑⇒|Av| ↑⇒ •R ↑⇒|RCL| ↑⇒ slow switching •gm ↑⇒|W| ↑⇒ big transistor – (slow switching at input) Trade-off between speed and noise margin. During pull-up we need: • High current for fast switching • But also high incremental resistance for high noise margin. ⇒ use current source as pull-up 6.012 Spring 2007 Lecture 12 3 2. NMOS inverter with current-source pull-up I—V characteristics of current source: iSUP + 1 ISUP r i oc vSUP SUP _ vSUP Equivalent circuit models : iSUP + ISUP r r vSUP oc oc _ large-signal model small-signal model • High current throughout voltage range vSUP > 0 •iSUP = 0 for vSUP ≤ 0 •iSUP = ISUP + vSUP/ roc for vSUP > 0 • High small-signal resistance roc. 6.012 Spring 2007 Lecture 12 4 NMOS inverter with current-source pull-up Static Characteristics VDD iSUP VOUT VIN CL Inverter characteristics : iD = V 4 3 VIN VGS I + DD SUP roc 2 1 = vOUT vDS VDD (a) VOUT 1 2 3 4 VIN (b) High roc ⇒ high noise margins 6.012 Spring 2007 Lecture 12 5 PMOS as current-source pull-up I—V characteristics of PMOS: + S + VSG _ VSD G B − _ + IDp 5 V + D − V + G − V − D − ID(VSG,VSD) (a) = VSG 3.5 V 300 V = V + V = V − 1 V 250 (triode SD SG Tp SG region) V = 3 V 200 SG − IDp (µA) (saturation region) 150 = VSG 25 100 = 0, 0.5, VSG 1 V (cutoff region) V = 2 V 50 SG = VSG 1.5 V 12345 VSD (V) (b) Note: enhancement-mode PMOS has VTp <0.
    [Show full text]
  • Chapter 10 Differential Amplifiers
    Chapter 10 Differential Amplifiers 10.1 General Considerations 10.2 Bipolar Differential Pair 10.3 MOS Differential Pair 10.4 Cascode Differential Amplifiers 10.5 Common-Mode Rejection 10.6 Differential Pair with Active Load 1 Audio Amplifier Example An audio amplifier is constructed as above that takes a rectified AC voltage as its supply and amplifies an audio signal from a microphone. CH 10 Differential Amplifiers 2 “Humming” Noise in Audio Amplifier Example However, VCC contains a ripple from rectification that leaks to the output and is perceived as a “humming” noise by the user. CH 10 Differential Amplifiers 3 Supply Ripple Rejection vX Avvin vr vY vr vX vY Avvin Since both node X and Y contain the same ripple, their difference will be free of ripple. CH 10 Differential Amplifiers 4 Ripple-Free Differential Output Since the signal is taken as a difference between two nodes, an amplifier that senses differential signals is needed. CH 10 Differential Amplifiers 5 Common Inputs to Differential Amplifier vX Avvin vr vY Avvin vr vX vY 0 Signals cannot be applied in phase to the inputs of a differential amplifier, since the outputs will also be in phase, producing zero differential output. CH 10 Differential Amplifiers 6 Differential Inputs to Differential Amplifier vX Avvin vr vY Avvin vr vX vY 2Avvin When the inputs are applied differentially, the outputs are 180° out of phase; enhancing each other when sensed differentially. CH 10 Differential Amplifiers 7 Differential Signals A pair of differential signals can be generated, among other ways, by a transformer.
    [Show full text]