Experiment 10 - Multistage Amplifiers

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Procedure FIGURE 1. Cascode Amplifier with Current Source Supply V CC I SUP 10µF v OUT M3501 250kΩ Q 2 V BIAS 1 MΩ v IN Q 1 M3501 3.0 Procedure 3.1 The Cascode The Cascode circuit is nothing more than a Common Emitter - Common Base (or Common Source - Common Gate) cascade. Figure 1 above shows a simplified cascode with a current source load. The 2 port model for the cascode is shown below FIGURE 2. 2-Port Representation of Cascode R in G m R out R in -i in2 R out R L Common Emitter Stage Common Base Stage 2 of 5 Experiment 10 - Multistage Amplifiers
Procedure In your lab, the cascode circuit will include extra biasing circuits as shown in figure 3. These circuits make use of DC feedback (coupled through the external low-pass filter), in order to stabilize this high-gain circuit. After simplification, figure 3 reduces to the basic cascode amplifier in figure 1. 1. Set up the circuit from Lab Chip 4 as shown in figure 3. Let R Bias be 10 kΩ.and C in be 10 µF. Let V CC be set at 5V. Note: The user just needs to furnish the external elements in the box. (the elements in the dashed “boxes” in figure 3). 2. Determine the bias current and DC voltage at V OUT . Using figure 3, what are the maximum and minimum DC voltages that V OUT can swing to while keeping all the devices in the forward active region. Compare with measurements of output clipping levels. 3. Using the oscilloscope, find the gain v out /v in . Use a 5 kHz sine wave with an amplitude of 200 mV. If the signal at the output is clipped, decrease the input amplitude until no clipping occurs. 4. Calculate the input resistance and output resistance for the cascode. Using the calculated value of the input resistance, you can calculate how much of the input voltage is attenuated. Determine the gain of the cascode v out /v in . How does the cascode compare to the Common Emitter in terms of input resistance, output resistance and voltage gain? Optional: measure the input and the output resistances. FIGURE 3. Bipolar Cascode with DC Feedback Biasing (Lab Chip 4) Off-chip components: input signal, setting DC operating point of cascode VCC PIN 28 Q 5 Q M3511 6 M3511 I BIAS R Bias R 5 M3511 250Ω Q 4 PIN 17 v OUT R 4 Q 7 M3501 R 250Ω S = 1 MΩ C PIN 16 v in in M3501 M 9 Q 3 R R lp = 5MegΩ PIN 15 v 3 IN 250Ω PIN 19 M Q 2 2 Q 1 PIN 18 M3501 C R R 2 = 90kΩ 1 = 90kΩ M3501 lp = 10u R 6 = 3.75kΩ GND PIN 14 Experiment 10 - Multistage Amplifiers 3 of 5
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Page 5: Optional Experiments 4. Draw the 2

Experiment 10 - Multistage Amplifiers

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