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White Noise Generator Circuitry and Analysis

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White Noise Generator Circuitry and Analysis UNIVERSITY OF TEXAS AT SAN ANTONIO White Noise Generator Circuitry and Analysis David Sanchez 8/4/2008 Table of Contents Introduction .................................................................................................................................................. 3 Overview of the Circuit ................................................................................................................................ 3 Circuit Subsets .............................................................................................................................................. 4 Noise Generation Stage ........................................................................................................................ 4 Amplification of the Noise Signal .......................................................................................................... 6 Active Low-Pass Filter ........................................................................................................................... 7 Audio Output Stage............................................................................................................................... 8 Analysis ......................................................................................................................................................... 9 Discussion ................................................................................................................................................... 11 Conclusion .................................................................................................................................................. 12 Works Cited ................................................................................................................................................ 13 Electronic Circuits II Project – White Noise Generator Page 2 Introduction A white noise generator is just that – a circuit that produces white noise. White noise is essentially just distortion whose amplitude is constant through a wide frequency range. It is often produced by a random noise generator in which all frequencies are equally probable, just as white light is composed of all the colors of the visible light spectrum. The human hearing range is from approximately 20Hz to 20,000Hz. In this range, the human ear is more sensitive to the higher frequencies. Due to the fact that it incorporates all sound frequencies - from low, deep sounds to very high sounds - it has a very beneficial noise cancelling or masking effect. This noise finds applications in the medical, social, and technological fields. It is a gentle tone that can be found in nature, and the actual sound produced is comparable to rainfall or ocean waves. Overview of the circuit The circuit that produces white noise is fairly simple in nature. It consists of four stages or fragments – noise generation, signal amplification, low-pass filter, and audio output stage. A flow chart of the circuit is shown in Figure 1. The circuit uses all discrete parts that are both active and Active Noise Circuitry to Amp Low-pass Generation Speaker Filter passive. The active devices are the LF411 Figure 1 and LM386 operational amplifiers and the passive components are the resistors and capacitors. There are no inductors in this circuit. We decided to use op-amps for amplification because of the many advantages they have over discrete transistors, the most noticeable are “high efficiency, high gain, low standby power, low component count, small size and, of course, low cost” (Martell). The noise is generated from a pair of npn bipolar junction transistors that are tied Electronic Circuits II Project – White Noise Generator Page 3 together at their base terminals. This basically creates a zener diode and it is biased in the reverse breakdown region of operation. Being operated in this region, the pn junction starts to exhibit the zener breakdown phenomenon, and as such produces shot noise and creates a low-level, constant amplitude distortion signal. The noise generation stage is ac coupled to the amplification stage so as to pass the distorted signal but block the dc signal. The amplifier is set up in an inverting configuration which uses a negative feedback loop. This negative feedback helps to stabilize the output even further and helps to protect the signal from any spike that might occur. The gain of this amplifier stage is Av=100, and is 180˚ out of phase, as shown in the next section. The output of the amplification stage is then passed through a low-pass filter (LPF). Since human hearing ranges from 20Hz to 20kHz, the filter is design to pass these first 20kHz and block the higher (useless) frequencies. The cutoff frequency was design to be approximately 13kHz, with a -40dB/decade decrease thereafter. The passed signal from the LPF then enters an audio output stage. This stage basically amplifies the signal to a level that can be output through a speaker. The next section describes each of the stages in more depth and shows the circuit schematic for each fragment. Circuit Subsets Noise Generation Stage The first stage in our circuit is noise generation, where the constant power output is produced. We decided to generate noise with the zener breakdown phenomenon. Zener breakdown occurs when a zener diode is run in the reverse- breakdown region of operation. This usually occurs when approximately -1mA of current is passed through the diode. At this current level the zener diode enters Electronic Circuits II Project – White Noise Generator Page 4 reverse breakdown and the current through it drops rapidly while the voltage across it remains relatively constant. This voltage level is termed zener voltage and is represented by VZ. The I-V plot showing this phenomenon is shown in Figure 2. The noise generated while operating a zener diode in this region is based on the Figure 2 avalanche breakdown that occurs in the pn junction. In our circuit we actually did not use a zener diode but instead two npn bipolar transistors. These two transistors are tied together at their bases and connected to the same power supply. One of the BJTs is connected to the power supply at its collector terminal and tied to ground at the emitter. The other BJT is connected to the power supply at its emitter terminal and the collector terminal is floating. This essentially creates a pn junction all the same as a zener diode. The next step in the generation process was to 4.7k make sure that we were operating the transistors (pn junction) 15V in the reverse breakdown region. This was accomplished by Vsupply applying a +15VDC supply as power. To protect the power 1uF Q1 Q2 supply we put a 4.7k resistor in series with the transistors. We also put a 1uF capacitor from +15V to ground as a 0 0 blocking cap. This basically makes up the noise generation Figure 3 stage of the circuit. The circuit schematic of this stage is shown in Figure 3. Electronic Circuits II Project – White Noise Generator Page 5 Amplification of Noise Signal The next step in white noise generation is to amplify the very low noise that is produced by the transistors running in reverse breakdown. This was accomplished by using an operational amplifier with negative feedback. We decided to use a LF411 as the amplifier because of its extremely high open-loop gain (~250,000) and high input impedance (>106 Ω). Both of these are large enough to consider infinite, therefore ideal op-amp 0 15V analysis was used. One of the major non-ideal U1 7 3 5 + V+ B2 1k 6 characteristic of this amplifier is that the output 0 OUT 2 1 - V- B1 1uF 1k of the circuit cannot go past the power supply LF411 4 100k rails, plus-or-minus fifteen volts in our case. 15V This is okay though because the noise 0 generated and output is orders of magnitude Figure 4 smaller than 15V, and therefore we ignored this shortcoming. Using negative feedback we were able to control the gain of the 411 externally. We wanted a gain of approximately 100 so we used a 1kΩ resistor at the inverting terminal of the op-amp and used a 100kΩ resistor from the output (pin 6 for the LM411) back to the inverting terminal. In doing so we created an inverting amplifier with the gain of: AV = –R2/R1 = -100k/1k = 100 The non-inverting terminal of the op-amp was grounded through a 1kΩ. The circuit schematic of this stage is shown in Figure 4. Electronic Circuits II Project – White Noise Generator Page 6 Active Low-Pass Filter Once the low-level noise is amplified it needs to be tailored to output over the frequencies of interest, 0Hz to 20kHz in our case (human hearing range). To accomplish this we designed an active low pass filter with a cutoff frequency of 13kHz. It is termed “active” because it uses an op-amp instead of just passive components such as resistors, capacitors, and inductors. The circuitry of this LPF is shown in Figure 5. The op-amp we used to 200pF implement this filter was again an LF411. This 0 15V time however it had both positive and negative U2 7 3 5 + V+ B2 62k 62k feedback, allowing us to customize the output 6 OUT 2 1 200pF - V- B1 LF411 characteristics. The first decision we had to make 4 R9 1k 15V was whether or not to have gain with this amplifier. 0 Since we had a dedicated stage just for R10 0 1k amplification of the noise, we decided to set it up as a voltage follower, giving us unity gain over the 0 frequency range of interest. It also provides high Figure 5 input impedance and very low output impedance. At low frequencies (<<13kHz) the filter passes the noise generated
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