Electrical Engineering

Experiment 9 - Source Follower Amplifier Page 89

Figure 9.1 - Bootstrap Source Follower Amplifier

EXPERIMENT 9

Source Follower Amplifier

Objective

The purpose of this experiment is to become familiar with the source follower FET amplifier specifically the type referred to as a bootstrap source follower amplifier.

1.0 Discussion

The source follower FET amplifier has the same configuration as the emitter follower BJT amplifier. Some of the characteristics of the source follower are:

1. Voltage gain less than or equal to unity 2. High current gain 3. Very high input impedance 4. Low output impedance 5. Output in phase with input The Bootstrap source follower (Figure

9.1) is a special variation of the source follower in which the bias is developed across part of the source resistor. This eliminates the need for a capacitor bypass across RS2 and thus reflects a much larger input impedance than normally can be obtained when only R1 is used. The design takes advantage of the FETs intrinsic high impedance without requiring a high value for the gate resistor, RG. The larger RG is, the more leakage current there will be, and gate leakage current causes instability.

Different FETs exhibit different leakage currents which, in turn, cause different currents in RG. Thus, the voltage drop across RG is not constant, and the amplifier gain is not constant. If RG is large, this problem can have a significant effect on amplifier stability.

1.1 ac Operation

The expressions for voltage gain, current gain, and input impedance are derived using the ac equivalent circuit in Fig. 9.2.

Page 90 Experiment 9 - Source Follower Amplifier

Figure 9.2 - ac Equivalent Circuit for Source Follower

The expressions are presented here in final form only: Voltage Gain:

Current Gain:

Input Resistance

Also, the value of the RG resistor is given by:

1.2 dc Operation

The gate-source loop equation is:

(assume the I in RG is zero) (9.5)

The drain-source loop equation is: (9.6)

1.3 Bootstrap Amplifier Design

1. Choose the Q point on the most linear portion of the characteristic curve plot. (This information may be provided for you).

Experiment 9 - Source Follower Amplifier Page 91

2. Determine gm either from the specs or using .

In these problems, you will be given VDD, RL, and Zin.

3. Use Equation 9.5 to find RS1.

4. Use Equation 9.6 to find RS2.

5. Choose RG for the Zin criterion using Equation 9.4.

6. Calculate Ai from Equation 9.2.

7. Find Av from Equation 9.1 or the gain-impedance formula.

2.0 Preparatory Work

The following problems require the use of the circuit in Figure 9.1.

2.1 If VDD = 12 (V), RL = 1 kS, and Rin = 1 MS, find RS1, RS2, and RG. Choose the Q point of VDS = 6 (V), ID = 6.1 mA, VGS = 0.8(V) and gm = 3.33 mS

-1. Calculate the voltage and current gain.

2.2 If a resistor were placed in the drain circuit, what effect would it have on the operation of the bootstrap circuit?

2.3 Why will gate leakage current cause instability?

2.4 Why is the bootstrap circuit used instead of a standard source follower?

2.5 If VDD = 20 (V), RL = 5 kS, and Rin = 400 kS, find RG, Ai, and Av for the circuit with Q point coordinates: VDS = 10 (V), VGS = 1 (V), ID = 2 mA, and 1/gm = 500 S.

2.6 Which parameters would change in Problem 2.5 if Rin = 100 kS?

2.7 If a 1 kS RD resistor were placed in the circuit in Problem 2.5, what effect would it have on the amplifier gain? On the value of RG?

Page 92 Experiment 9 - Source Follower Amplifier

Figure P9.1

3.0 Procedure

3.1 Wire the circuit in Fig. P-9.1 as determined from calculation when the Q- point has been selected.

*Pick a Q point from the characteristic curve when VDS = 6(V). Determine R1, R2 and RG. Select resistors as close as possible from your kit.

Use FET 2N5951 and the values for RG, RS1, and RS2 selected from your kit. Use a decade box for RTest.

3.2 Measure VDS, VGS, and ID. Compare the measured values with those calculated in 3.1. If there is a large discrepancy (greater than 20%) connect a variable resistor in place of RS2 to adjust the Q point position. Record the final (adjusted) values of VDS, VGS and ID.

3.3 Set RTEST = 1 kS. Apply a 1 kHz signal to Vin and increase signal amplitude until distortion just begins at the output, Vout. At the point just before distortion begins, measure and record input voltage (both Va and Vin) output voltage, voltage gain, and current gain.

*To find current gain, use

output current =

input current =

3.4 Repeat step 3 with the applied signal frequency equal to: 50 Hz, 100 Hz, 500 Hz, 10 kHz, and 20 kHz.

3.5 Set the frequency to 1 kHz and

Experiment 9 - Source Follower Amplifier Page 93

Figure P9.2

Figure P9.3

measure the input impedance. The usual voltage division method will not work in this case because the FET input impedance is so high. In a normal circuit, you can use

The scope impedance is so high with respect to Zin that Zscope2Zin is very nearly Zin. When Zscope is approximately equal to Zin (as in the FET case), the input impedance you measure with the voltage division method is Zscope2Zin.

We have, Zin measured = Zin real2Zscope. The technique is as follows: Use voltage division to find Zin measured (equal Zin in Figure P-9.2). Use voltage division with only the scope as the load and measure

Page 94 Experiment 9 - Source Follower Amplifier

Zscope.Use your measured values of Zin and Zscope to solve this equation for the actual Zin(Zin real).

3.6 Repeat step 5 with the applied signal frequency equal to: 50 Hz, 500 Hz, 5 kHz, and 20 kHz.

4.0 Analysis

4.1 Compare the selected VDS, VGS and ID to those values measured in your circuit. Note the value of RS2 used to achieve your final Q point. Is this value different than your calculated RS2?

4.2 Compare the voltage gain, current gain, and input impedance at 1 kHz to the values calculated. Find the percent error in your values.

4.3 Plot voltage gain, current gain, and input impedance as functions of frequency. (Plot each variable separately).