Lab Report 4

Lab Report 4 - Scott Smith ENEE306-0104 8 November 2005 Lab...

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Scott Smith ENEE306-0104 8 November 2005 Lab Report 4 4.1 – Introduction All amplifiers have some type of frequency response at all frequencies. What we have covered so far is called the mid-band gain. There is a low frequency where the voltage gain of the amplifier increases to the mid-band gain at a rate of approximately 20 dB/dec and a high frequency where the voltage gain of the amplifier drops off at a rate of approximately 20 dB/dec . This typical frequency response is shown in figure 1. In this lab we will explore the methods used to calculate the frequencies that this phenomenon occurs, called poles. We will discuss the brute-force approach, short circuit time constant approximation, Miller’s approximation, and open circuit time constant analysis. Figure 1 4.2 – Low Frequency Brute Force Approach We already know a lot about the CE amp from lab 2. However, now we will take the effect of the coupling capacitors into account. Here ω j s = . Gain of the 1 st stage: 1 1 1 sC in in in R R v v + = (1) Gain of the CE amp: m g E C R R A 1 + = (2)
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Smith, 0104 Gain of the 3 rd stage: 2 1 1 sC o L L o R R R Av v + + = (3) AC Coupled CE Amplifier v in C 1 + v out - R 2 R 1 R C R E R L C 2 V CC Figure 2 Large Signal Equivalent Circuit + Av 1 - C 1 + v out - R in R o R L C 2 + v in - + v 1 - Figure 3 The total gain of the circuit in figure 2 is + + + + = 2 1 1 1 1 sC C L L g E C sC in in in out R R R R R R R v v m (4) Simplifying (4), we get () + + + = + 2 1 1 1 1 0 0 || C R R C R g E L C in out C L in m s s s s R R R v v (5) 2
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Smith, 0104 where the value of s represents the poles in radians per second. Stages 1 and 3 act as high pass filters, letting higher frequencies through while blocking lower ones. This is why the coupling capacitors are used to isolate the DC bias from the source and load. 4.3 – CE Amp with Emitter Bypass Capacitor The resistor placed at the emitter facilitates DC biasing. However, the emitter resistance negatively impacts the voltage gain. By placing a large capacitor in parallel with the emitter resistance, this removes the adverse influence on the voltage gain. The circuit behaves just as a CE with grounded emitter when at Midband. However, computing the poles through brute force would require a computer program to solve it. There are other methods that are useful for approximate and quick calculations. 4.4 – Short Circuit Time Constant Approximation (SCTCA) This method is a useful approximation of the dominant low frequency pole. This is the point in the frequency domain at which the voltage gain will start to drop. This approximation simplifies the brute force method in the following way.
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This note was uploaded on 05/04/2008 for the course ENEE 306 taught by Professor Goldsman during the Spring '04 term at Maryland.

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Lab Report 4 - Scott Smith ENEE306-0104 8 November 2005 Lab...

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