Module: Electronics II Module Number: 650321 Electronic Devices and Circuit Theory, 9th ed., Boylestad and Nashelsky Philadelphia University Faculty of Engineering Communication and Electronics Engineering Amplifier Circuits-III Operational Amplifiers (Op-Amps): An operational amplifier, or op-amp, is a very high gain differential amplifier with high input impedance and low output impedance. Typical uses of the operational amplifier are to provide voltage amplitude changes (amplitude and polarity), oscillators, filter circuits, and many types of instrumentation circuits. An op-amp contains a number of differential amplifier stages to achieve a very high voltage gain. - Symbol and Terminals: The following Figure shows a basic op-amp with two inputs and one output as would result using a differential amplifier input stage. Each input results in either the same or an opposite polarity (or phase) output, depending on whether the signal is applied to the plus (+) or the minus (-) input. To illustrate what an op-amp is, let's consider its ideal characteristics. A practical op- amp, of course, falls short of these ideal standards, but it is much easier to understand and analyze the device from an ideal point of view. These two considerations are clearly shown in the Figure Below. Firstly, the ideal op-amp has - infinite voltage gain, - infinite bandwidth, - it has an infinite input impedance (open) so that it does not load the driving source, - a zero output impedance. Lecturer: Dr. Omar Daoud Part III 1 Module: Electronics II Module Number: 650321 Electronic Devices and Circuit Theory, 9th ed., Boylestad and Nashelsky The input voltage, Vin appears between the two input terminals, and the output voltage is AvVin as indicated by the internal voltage source symbol. Secondly, characteristics of a practical op-amp are: - very high voltage gain, - very high input impedance, - low output impedance, and - wide bandwidth. - Internal Block Diagram: A typical op-amp is made up of three types of amplifier circuit: a differential amplifier, a voltage amplifier, and a push-pull amplifier, as shown in the Figure below. A differential amplifier is the input stage for the op-amp; it provides amplification of the difference voltage between the two inputs. The second stage is usually a class A amplifier that provides additional gain. Some op-amps may have more than one voltage amplifier stage. A push-pull class B amplifier is typically used for the output stage. Lecturer: Dr. Omar Daoud Part III 2 Module: Electronics II Module Number: 650321 Electronic Devices and Circuit Theory, 9th ed., Boylestad and Nashelsky The Differential Amplifier Input Stage The term differential comes from the amplifier's ability to amplify the difference of two input signals applied to its inputs . Only the difference in the two signals is amplified; if there is no difference, the output is zero . A basic differential amplifier circuit and its symbol are shown in the Figure. The transistors (Q1 and Q2) and the collector resistors (Rc1 and Rc2) are carefully matched to have identical characteristics. Notice that the two transistors share a single emitter resistor, RE. The Differential Amplifier Operation: - Assume both bases are connected to ground, - The emitter voltage will be -0.7 V because the voltage drops across both base- emitter junctions are equa1. - The emitter currents are equal (IE1 = IE2) and each is one-half of the current through RE. - The collector currents are both equal and are approximately equal to the emitter currents. Because the collector currents are the same, the collector voltages are also the same, which reflects the zero difference in the input voltages (both bases are at 0V. - If the base of Q1 is disconnected from ground and connected to a small positive voltage, - Ql will conduct more current because the positive voltage on its base causes the emitter voltage to increase slightly. - Although the emitter voltage is a little higher, the total current through RE is nearly the same as before. - The emitter current is now divided so that more of it is in Ql and less in Q2. - As a result, the collector voltage of Q1 will decrease and the collector voltage of Q2 will increase, reflecting the difference in the input voltages (one is 0 V and the other at a small positive value). Lecturer: Dr. Omar Daoud Part III 3 Module: Electronics II Module Number: 650321 Electronic Devices and Circuit Theory, 9th ed., Boylestad and Nashelsky - If the base of Ql is placed back at ground and a small positive voltage is connected to the base of Q2, - Q2 will conduct more current, - Ql will conduct less, - The emitter current is now divided so that more of it is in Q2 and less in Q1, - As a result, the collector voltage of Q1 will increase and the collector voltage of Q2 will decrease. - The differential amplifier exhibits three modes of operation based on the type of input signals. These modes are single-ended, differential, and common. Since the differential amplifier is the input stage of the op-amp, the op-amp exhibits the same modes. Op-Amp Input Modes and Parameters 1) Input Signal Modes Single-Ended Mode: one input is grounded and a signal voltage is applied only to the other input. - In the case where the signal voltage is applied to the inverting input, an inverted, amplified signal voltage appears at the output. - In the case where the signal is applied to the noninverting input with the inverting input grounded, a non inverted, amplified signal voltage appears at the output. Differential Input Mode: Two opposite-polarity (out-of-phase) signals are applied to the inputs. This type of operation is also referred to as double- ended. The amplified difference between the two inputs appears on the output. Lecturer: Dr. Omar Daoud Part III 4 Module: Electronics II Module Number: 650321 Electronic Devices and Circuit Theory, 9th ed., Boylestad and Nashelsky Common Mode Input: Two signal voltages of the same phase, frequency, and amplitude are applied to the two inputs. When equal input signals are applied to both inputs, they cancel, resulting in a zero output voltage. This action is called common-mode rejection. Its importance lies in the situation where an unwanted signal appears commonly on both op-amp inputs. 2) Input Parameters Common Mode Rejection Ratio: Common-mode rejection means that this unwanted signal will not appear on the output and distort the desired signal. Common-mode signals (noise) generally are the result of the pick-up of radiated energy on the input lines, from adjacent lines, the 60 Hz power line, or other sources. The measure of an amplifier's ability to reject common-mode signals is a parameter called the CMRR (common-mode rejection ratio). where Ad = differential gain of the amplifier Ac = common-mode gain of the amplifier Vd = difference voltage Vc = common voltage Ex. Calculate the CMRR for the circuit measurements shown in Fig. Lecturer: Dr. Omar Daoud Part III 5 Module: Electronics II Module Number: 650321 Electronic Devices and Circuit Theory, 9th ed., Boylestad and Nashelsky Solution: Ex. Determine the output voltage of an op-amp for input voltages of Vi1 = 150 V, Vi2 =140 V. The amplifier has a differential gain of Ad = 4000 and the value of CMRR is: (a) 100. (b) 105. Solution: Lecturer: Dr. Omar Daoud Part III 6 Module: Electronics II Module Number: 650321 Electronic Devices and Circuit Theory, 9th ed., Boylestad and Nashelsky Common mode Input voltage range: It is the range of input voltages which, when applied to both inputs, will not cause clipping or other output distortion. Many op-amps have common-mode input voltage ranges of ±10 V with dc supply voltages of ±15 V. Input Impedance: Two basic ways of specifying the input impedance of an op-amp are the differential and the common mode. The differential input impedance is the total resistance between the inverting and the noninverting inputs (measured by determining the change in bias current for a given change in differential input voltage). The common-mode input impedance is the resistance between each input and ground (measured by determining the change in bias current for a given change in common-mode input voltage). Input Offset Voltage: The ideal op-amp produces zero volts out for zero volts in. In a practical op-amp, however, a small de voltage. VOUT(error), appears at the output when no differential input voltage is applied (causes a slight mismatch of the base-emitter voltages of the differential amplifier input stage of an op-amp) . The input offset voltage, Vos, is the differential dc voltage required between the inputs to force the output to zero volts (Typical values of in the range of 2 mV or less, while it is 0V in ideal cases). Input Offset Current: Ideally, the two input bias currents are equal, and thus their difference is zero. In a practical op-amp, however, the bias currents are not exactly equal. The input offset current, Ios, is the difference of the input bias currents, expressed as an absolute value (Actual magnitudes of offset current are usually at least an order of magnitude (ten times) less than the bias current). VIRIRIIRos 1 in 2 in 1 2 in VIRos os in VOUT (error) AIRv os in Lecturer: Dr. Omar Daoud Part III 7 Module: Electronics II Module Number: 650321 Electronic Devices and Circuit Theory, 9th ed., Boylestad and Nashelsky Slew Rate: The maximum rate of change of the output voltage in response to a step input voltage is the slew rate of an op-amp. The slew rate is dependent upon the high-frequency response of the amplifier stages within the op-amp. A pulse is applied to the input and the resulting ideal output voltage is indicated as below.