UNIVERSITY OF CALIFORNIA AT BERKELEY College of Engineering Department of Electrical Engineering and Computer Sciences EE105 Lab Experiments Experiment 8 Multi stage Amplifiers 1 Objective Often a single stage transistor amplifier may not provide enough gain or input output impedance for a desired application To remedy this we can cascade amplifier stages to form a multi stage amplifier with the desirable gain or impedance properties In this lab we will examine the cascode amplifier and another multi stage amplifier formed by cascading a common emitter and common collector amplifier 2 Materials Component 2N4401 NPN BJT 2N4403 PNP BJT 51 k resistor 20 k resistor 100 resistor 51 resistor 10 k potentiometer 10 F capacitor 100 F capacitor Quantity 4 2 2 1 2 1 1 1 1 Table 1 Components used in this lab 3 Procedure 3 1 Cascode Amplifier 1 Construct the cascode amplifier Q1 and Q2 with current mirror bias Q3 and Q4 as shown in Figure 1 a Use two 51 k resistor in parallel to make a 25 5 k resistor for RREF and a 51 resistor for RS Set VBIAS2 to 1 5 V 2 Use the function generator to generate a 1 kHz 20 mV peak to peak sinusoidal signal with a DC offset of around 580 mV to 650 mV you may have to adjust the offset after connecting the signal to the amplifier to ensure you get a clean output signal Use this signal as vIN 3 Measure IBIAS and the DC voltage at vOUT 4 Using the oscilloscope plot both the input vIN and the output vOUT Sketch the waveforms you observe 5 Why is vOUT not sinusoidal 6 What is the peak to peak voltage of the output waveform What are the gain and phase of vOUT vIN 1 3 2 PROCEDURE VCC 12 V Q3 VCC 12 V Q4 Q3 Q4 vOUT VBIAS2 vOUT Q2 VBIAS2 Q2 C RS RREF Q1 IBIAS vIN RS Q1 RL vL RREF IBIAS vIN a b Figure 1 a Cascode amplifier test setup b Cascode amplifier test setup with load resistance 7 Now add a 10 F capacitor to the node vOUT and a 20 k resistor from the capacitor to ground as shown in Figure 1 b This resistor will act as a load to the amplifier 8 What is the peak to peak voltage of the output waveform at vL with the load resistor What is the gain of the amplifier with the resistive load 9 Estimate the output resistance of this amplifier using the peak to peak voltages measured 3 2 Common Emitter Common Collector Multi stage Amplifer From the previous lab exercises you tried using a speaker as a load to a common emitter amplifier However the common emitter amplifier delivers very little voltage gain to the speaker because of the huge impedance mismatch between the amplifier and speaker In this section of the lab you will cascade a low output impedance common collector amplifier to the output of a common emitter amplifier as a voltage buffer to drive a low impedance speaker 1 Before you begin use the function generator to apply a sinusoidal signal with peak to peak voltage of 40 mV and a frequency of 1 kHz directly to the speaker Can you hear anything 2 Construct the cascaded amplifiers as shown in Figure 2 Set the potentiometer to around 8 k for RC For the rest of the circuit let RS 51 RREF 200 use two 100 resistors in series and C 100 F 3 Use the function generator to generate a 40 mV peak to peak 1 kHz frequency sine wave with a DC offset of around 540 mV to 600 mV Use this signal as vIN Now try to maximize the gain of the amplifier by increasing the resistance of the potentiometer for RC Can you hear anything now Feel free to try out other frequencies to observe how the speaker responds to various frequencies 4 Measure IBIAS1 IBIAS2 and the DC voltages at vOUT 1 and vOUT 2 5 Measure VBE of Q2 Is the DC voltage at vOUT 1 enough to bias Q2 in the forward active region 6 Using the oscilloscope plot both the input vIN and the output vOUT 2 Sketch these waveforms 3 3 PROCEDURE VCC 5 V RC IBIAS1 RREF Q2 vOUT 1 RS Q1 IBIAS2 vOUT 2 C vIN Q3 Q4 Figure 2 Multi stage amplifier test setup 7 Measure the gain and phase of vOUT 2 vIN 8 Now increase the DC offset of the input waveform to 620 mV What happens to the waveform at vOUT 2
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