Multi Stage Transistero Amplifier

University of Sharjah

College of Engineering

Experiment No. 6 –Multi-stage and feedback amplifier Name/ID:

Date:

Mark:

Objectives: i.) ii.) iii.)

At the end of this experiment the student should be able to: Measure and analyze the performance of multi-stage amplifiers. Understand the effect of loading on the gain. Understand the effect of negative feedback on the properties of amplifiers

Introduction The single-stage BJT CE amplifier was shown to provide large voltage gain and moderate input and output resistances. A good voltage amplifier should have large gain, high input resistance (Ri) and low output resistance (Ro). The importance of Ri 

R





R



i L and Ro is clear from the overall gain equation, A =  R + R ( − g m Ro )  R + R . S  L   i  o

So, if Ri is not high enough compared to Rs, the gain will be reduced. Similarly, if Ro is not low enough compared to the load, RL, the gain will be reduced. In a single-stage BJT amplifier, if Ri is increased by including an emitter degeneration RE, the gain will be reduced by a factor of approximately gmRE. Similarly, since the gain depends directly on Ro, reducing it will automatically reduce the gain. Therefore, additional stages are required to optimize the amplifier gain, Ri and Ro. In general, at least three stages are needed, an input stage, a gain stage and an output stage. Feedback is basically a circuit technique, where a link is created between the output and the input. This establishes a feedback loop and results in improved amplifier properties. In this experiment, a two-stage BJT amplifier will be studied. The amplifier consists of a BJT CE amplifier gain stage followed by another CE amplifier output stage. For cascaded CE stages, the input impedance of the second stage comprises the load for the first stage. Therefore, for cascade amplifiers there is a loading effect on the preceding stage. In other words, the unloaded voltage gain is higher than the loaded voltage gain. In this experiment students 28

University of Sharjah

College of Engineering

will construct a cascaded CE amplifier to stabilize and boost the overall voltage gain and observe effect of loading. Also negative feedback will be applied to two CE BJT cascaded amplifiers and the effect on amplifier properties will be measured.

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University of Sharjah

College of Engineering Equipment & Components Required

Prototype board, power Supply Digital multimeter Function generator

capacitors 10 μF (four), 47μF (two) Resistors: 200k, 120k, 47k, 30k, 20k, 12k (two), 10k, 3.9k, 3k, 1k and 220. Transistors: Two Q2N3904 or BC107

BJT CE Amplifier Measurements Measure all resistors and connect the BJT CE amplifier shown in Fig. 1, Step 1

Figure 1 Step 2

BJT Biasing Point: Use the multimeter to measure the transistor biasing point. Measure the voltages (VB, VE, VC) and currents (IC, IE, IB). Analysis/Discussion: From your measured data: Verify that the transistor is in the active mode.

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University of Sharjah

College of Engineering

Step 3

Amplifier Gain: Apply a 10mV, 10kHz sine wave. Measure the voltage gain for the circuit as given in Fig. 1 Vs(p-p) = Vo(P-P) = A1 =

Step 4

Measure all resistors and connect the BJT CE amplifier shown in Fig. 2,

Fig. 2. Step 5

BJT Biasing Point: Use the multimeter to measure the transistor biasing point. Measure the voltages (VB, VE, VC) and currents (IC, IE, IB). Analysis/Discussion: From your measured data: Verify that the transistor is in the active mode.

Step 6

Amplifier Gain: Apply a 10mV, 10kHz sine wave. Measure the voltage gain for the circuit as given in Fig. 2. Vs(p-p) = Vo(P-P) = A2 =

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University of Sharjah Step 7

College of Engineering

Common – Emitter Common – Emitter cascade: Connect the BJT CE cascaded amplifiers shown in Fig. 3,

Fig. 3 Step 8

BJT Biasing Point: Use the multimeter to measure the transistors biasing point. Measure the voltages (VB, VE, VC) and currents (IC, IE, IB). Analysis/Discussion: From your measured data: Verify that both transistors are in the active mode.

Step 9

Amplifier Midband Gain and Bandwidth (With Feedback): a. With the feedback loop. Apply a 10mV, 10kHz sine wave. Verify operation in midband. b. Measure the midband gain A3 c. Measure the amplifier cutoff frequencies (fL and fH) and bandwidth (BW).

Step 10

Amplifier Midband Gain and Bandwidth (Without Feedback):: a. Open the feedback loop (disconnect either RF or CF) and apply a 10mV, 10kHz sine wave. Verify operation in midband. b. Measure the midband gain A4 c. Measure the amplifier cutoff frequencies (fL and fH) and bandwidth (BW).

Step 11

Discussion: 32

University of Sharjah

College of Engineering

1. Compare A4 to A1 * A2, what do you think should be the relation between them if there is no loading effect? 2. Discuss if there is loading from the second stage on the first stage. (Use results from step 10). 3. Compare the results for amplifier gain and bandwidth with and without feedback showing the effect of feedback on amplifier gain, BW.

Write a laboratory report including your simulated results and print out for all simulated graphs. Do not forget to answer the questions raised in the laboratory sheets in your laboratory report !!

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