Experiment 8 - Single Stage Amplifiers with Passive Loads - BJT

UC Berkeley EE 105

1.0 Objective

A typical integrated circuit contains a large number of transistors that perform many functions. The simplest way to analyze such a circuit is to regard each individual transistor as a stage and to analyze the circuit as a collection of single transistor stages. In this experiment, you will examine the behavior of some single-stage amplifiers with resistors supplying the bias current. You will measure properties such as voltage gain, input impedance and output impedance. From these measurements you should understand the relative trade-offs between amplifiers. Keep a copy of your write-up so you can compare data with lab9.

To show your understanding of the lab, your write-up should contain:

A table showing the input resistance, output resistance, and gain of the amplifiers

A discussion on trade-offs issues among the three parameters A_v, R_in, and R_out

A discussion explaining the advantages and disadvantages of the different amplifiers

2.0 Prelab

H & S: Chapters 8.1 - 8.2, 8.7, 8.9

Biasing will often present a problem when building amplifiers. Below is a npn bipolar transistor which will be configured as a common emitter amplifier. Usually, we would like to have the DC voltage V_OUT to be in the middle of the highest and lowest allowable voltage. For the circuit in Fig. 1 below, determine the proper voltage for V_OUT (to 2 significant digits) and determine the proper biasing voltage V_BIAS needed to achieve this. Use SPICE rather than hand calculations to confirm this. Hand calculations would be useful for a starting point in guessing what V_BIAS should be. Use the following parameters:

_F=100, V_AF =100 V, I_S = 1 X 10¹⁵ A, and V_CE(SAT) = 0.2V

FIGURE 1.

3.0 Procedure

For all the experiments in this lab, use a 5 kHz sine wave with an amplitude of
200 mV.

Make sure that both channels of the oscilloscope is calibrated since you will be comparing the signal at the output and comparing it with the input.

Grounding the circuit's ground to the metal base of the breadboard can give cleaner signals.

Do not trigger the oscilloscope in Vert Mode. You will lose phase information!

Most of the circuits here will contain special biasing circuits to set the collector currents of the npns or the drain currents of the FETs. The drain or collector currents will be equal to the current across a biasing resistor -- there is an internal circuit that is responsible which we will study in Exp. 11. The circuit below will illustrate this point.

FIGURE 2.

The user will provide R_BIAS across pin 28 and pin 27. The current across the resistor, I_BIAS, becomes the collector current I_C for the BJT. I_BIAS can be found by use of a voltmeter across R_BIAS., with I_BIAS equal to the voltage across the resistor divided by its resistance.

3.1 Common Emitter Amplifier

1. For the circuit in figure 3, adjust R_BIAS until V_OUT = 2.5V. (R_BIAS should be about 16 k) Measure the current through R_BIAS. Is it the same as the collector current? Also check V_BE and V_BC to verify that the transistor is in its forward action region of operation. Why is V_OUT = 2.5 V a good choice?

2. Using the oscilloscope, measure the small-signal voltage gain

FIGURE 3.

3.1.1 Input Resistance Measurement

1. Why is A_v2 lower than A_v1? What is the relationship if between A_v2 and A_v1? Note that the open-loop voltage gain of the amplifier is A_v1, not A_v2.

2. Using that relationship, you should be able to find the input resistance. In this case, the input resistance happens to be r.What is the value of r?

3.1.2 Output Resistance Measurement

1. Measure the amplitude at v_out. Connect the capacitor and the variable resistor to the output of the circuit as depicted in figure 4. Adjust the resistance until the amplitude at v_out is reduced by one-half. The value of the variable resistor is equal to the output resistance. Explain why this procedure measures the output resistance. Also explain the function of the capacitor.

FIGURE 4.

3.2 Common Collector Amplifier (Emitter Follower)

1. Figure 5 shows a common collector circuit with a current source bias. I_BIAS is the same current as through R_BIAS. Let R_BIAS be 100k, I_BIAS. What is the voltage gain

2. Find the input and output resistance using the methods from earlier sections. The current source is actually an npn transistor, so its output resistance is just r_o = V_A/I_C. Note that the voltage gain of the common collector configuration cannot exceed unity. Do your results agree? In what situation would one use a Common Collector?

FIGURE 5.

4.0 Optional Experiments

4.1 Common Emitter with Emitter Degeneration

1. Connect the circuit shown in figure 6. Why is there no biasing problem with this circuit? Repeat the procedures for the Common Emitter Amplifier. How do the values of voltage gain, input impedance, and output impedance compare to the corresponding values for the common emitter configuration?

FIGURE 6.