Exercise 1: Effect of Shunt Feedback on AC Gain

Transcription

1 Exercise 1: Effect of Shunt Feedback on AC Gain When you have completed this exercise, you will be able to understand the effect of shunt negative feedback on ac gain by using a typical shunt feedback circuit. You will verify your results with an oscilloscope. To understand the effect of shunt negative feedback in this cicuit, assume that there is a small positive change in input voltage (V i ). The positive change in V i increases the transistor base current, which, in turn, increases the collector current. As collector current (I c ) increases, what happens to the collector voltage (V c )? a. decreases b. increases c. 82 Festo Didactic P0

2 A portion of the collector voltage is fed back to the junction of R f and R i. Because the two changing voltages (V i and V c ) are opposite in phase, the negative-going change of V c opposes the initial positive change due to V i. Would the foregoing action hold true for a negative-going change in V i? a. no b. yes The effect of shunt negative feedback on gain stability can be demonstrated by the following equation, f A f )] Festo Didactic P0 83

3 Typically, A is much larger than A f A f where = R i f. The equation shows that the input (R i ) and feedback (R f ) resistors determine and A f. The gain is relatively independent of transistor characteristics or other circuit parameters. i is 12 k and R f is 192 k. From the equation, you can calculate the approximate gain (A f ). A f = R f i A f = 192 k A f = 16 Calculate the feedback factor ( ). = R i f (Recall Value 1) 84 Festo Didactic P0

4 shown. Do not connect feedback resistor R2 at this time. All ac oscilloscope measurements and accompanying calculations are peak-to-peak (pk-pk) values, unless stated otherwise. Use ac coupling and x10 probes for these measurements. Adjust the positive variable supply to Vdc. Festo Didactic P0 85

5 Adjust R3 to obtain 5.5 Vdc at the transistor collector. o ). Set the signal generator for a 3 V pk-pk o ), which is b ). Measure the input voltage (V b ) without feedback. V b = mv pk-pk (Recall Value 1) 86 Festo Didactic P0

6 Calculate gain (without feedback) from your set and measured values. V o (without feedback) = 3.0 V pk-pk V b (without feedback) = (Step 7, Recall Value 1) A = V o b A = (Recall Value 2) Connect shunt feedback resistor R2. Readjust R3 for 5.5 Vdc at the collector of Q1. Festo Didactic P0 87

7 Readjust the generator so that V o is 3 V pk-pk. Because gain with feedback now includes series input resistor R1, you must make measurements of gain from the input terminal (V i ) to the output terminal (V o ). i ). Measure the input voltage (V i ) with shunt feedback. V i = V pk-pk (Recall Value 3) Calculate gain (with feedback) from your set and measured values. V o (with feedback) = 3.0 V pk-pk V i = V pk-pk (Step 13, Recall Value 3) A f = V o i A f = (Recall Value 4) 88 Festo Didactic P0

8 Compare gain with feedback and without feedback. Gain without feedback (A) = (Step 8, Recall Value 2) Gain with feedback (A f ) = (Step 14, Recall Value 4) Did the gain increase or decrease when feedback was introduced? a. increase b. decrease Determine the circuit gain from the values of the input resistor and feedback resistor. A f A f = (Recall Value 5) A f (meas) = (Step 14, Recall Value 4) A f (calc) = (Step 16, Recall Value 5) Do your values for measured and calculated gain agree within tolerance? a. no b. yes feedback resistor (R f ) to the input resistor (R i ). A f = R f i Festo Didactic P0 89

9 1. circuit shown. Adjust R3 for 5.5 Vdc at the collector of Q1. 90 Festo Didactic P0

10 Set the sine wave generator for an input of 200 mv pk-pk (A f = V o i ). Which value is closest to yours? a b. 2.2 c. 1.5 d f ), which equation can you use? a. A f b. A f c. A f d. A f 3. The formula A f ]) shows the relationship between gain with feedback (A f ) and gain without feedback (A). What does the symbol in the formula represent? a. frequency cutoff point b. feedback ratio (R i f ) c. ratio of input to output (V o i ) d. circuit bandwidth Festo Didactic P0 91

11 4. If R2 is 6.8 k and R1 is 1.2 k f )? a. 7.5 b. 3.3 c. 2.0 d In this circuit, assume A f is 5.6 and the output voltage is 7.5 V pk-pk. What is the approximate value of V i? a. 3.3 V pk-pk b V pk-pk c V pk-pk d. half of V o Make sure all CMs are cleared (turned off) before proceeding to the next section. 92 Festo Didactic P0