Experiment 11 Multistage

W.T. Yeung, J.C. Rudell, and R.T. Howe

UC Berkeley EE 105 Fall 2003

1.0 Objective

In Exps. 7 and 8, you found the small signal properties of single stage amplifiers (e.g., , Common Collector, , etc.) Now you will see how these single stage amplifiers perform together. By cascading the single stage devices, new amplifiers with enhanced performance can be realized. You will also study the effects of loading between stages.

To show your understanding of the lab, your write-up should contain: • A discussion on how single stages interact together • A discussion on interstage loading based on 2-port models

2.0 Prelab

• H & S: Chapter 9 • For the Circuit in Fig. 1, hand-calculate the gain, input resistance and out- put resistance for a supply current of ISUP = 1 mA. β β (Use npn: n = 80, VAn = 50 V, - pnp: p = 30, VAp =15 V.) Given: the current-source supply in figure 1 has a small-signal resistance β roc = prop.

1 of 5 Procedure

FIGURE 1. Cascode with Supply

VCC

ISUP 10µF vOUT

M3501 250kΩ Q2

VBIAS 1 MΩ vIN Q1 M3501

3.0 Procedure

3.1 The Cascode The Cascode circuit is nothing more than a Common Emitter - (or Com- mon Source - ) 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

Rin Gm Rout Rin -iin2 Rout RL

Common Emitter Stage Common Base Stage

2 of 5 Experiment 11 Multistage Amplifiers Procedure

In your lab, the cascode circuit will include extra circuits as shown in Fig. 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, Fig. 3 reduces to the basic cascode amplifier in Fig. 1 for the frequencies we will be concerned with. Ω 1. Set up the circuit from Lab Chip 4 as shown in Fig. 3. Let RBias be 10 k .and Cin be µ 10 F. Let VCC be set at 3.5 V. Note: The user just needs to furnish the external ele- ments in the box. (the elements in the dashed “boxes” in Fig. 3).

2. Determine the bias current and DC voltage at VOUT. Using Fig. 3, what are the maxi- mum and minimum DC voltages that VOUT 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 vout/vin. Use a 5 kHz sine wave with an ampli- tude of 65 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 calcu- lated value of the input resistance, you can calculate how much of the input voltage is attenuated. Determine the gain of the cascode vout/vin. How does the cascode com- pare to the Common Emitter in terms of input resistance, output resistance and volt- age 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 = 3.5 V PIN 28

Q 5 Q6 M3511 M3511 I R BIAS Bias R5 Ω M3511 250 Q4 PIN 17 v OUT R4 Q7 Ω 250Ω M3501 RS = 1 M Cin PIN 16 vin M3501 M9 Q3 R = 5MegΩ PIN 15 R3 lp vIN 250Ω PIN 19 M2 Q2 Q PIN 18 1 C = 10u M3501 Ω Ω M3501 lp R = 90k R2 = 90k 1 R 3.75kΩ 6 = GND PIN 14

Experiment 11 Multistage Amplifiers 3 of 5 Procedure

Lab Tip

Find the DC voltage at vOUT (PIN 17) and make sure that Q3 and Q4 are not saturated. If they are, get a new chip.The circuit takes about 10 - 20 seconds to stabilize. Be patient.

5. Perform a SPICE analysis on the Cascode in Fig. 1 (the circuit in Fig. 3 is extra credit) and compare your results with simulation.

3.2 Cascading Stages 1. While leaving the Cascode intact, build the common emitter as shown in Fig. 4; do

not include the coupling capacitor yet. Set RBIAS (CEBIASP) to be a potentiometer and adjust it until the output is at 2.5V. (This is merely the same procedure as the

common emitter circuit in Exp. 7.) Find the bias current through RBIAS. Does the DC voltage at VOUT confirm the fact that IC = IBIAS? Find the gain of the Common Emit- ter. 2. Now cascade the two stages together with the use of the 10 µF coupling capacitor. What is its function? (Hint: look at the DC voltage at both sides of the capacitor) What would happen if the capacitor were not present? Change the amplitude of the sinusoid to 50 mV.

3. Find the gain for the cascade. Measure the gain vout1 / vin. Why is it reduced?

FIGURE 4. Cascode - Common Emitter Cascade

Lab Chip 4 V = 5V PIN 28 CC

PIN 28 VCC = 3.5 V Ω I is controlled by RC = 10k C RBIAS between CE_BIAS ISUP vOUT2 (PIN 27) and VCC. PIN 26 vOUT1 PIN 25

v M3501 PIN 17 10 µF b Q2

V PIN 19 BIAS Ground = PIN 14 Q1 Lab Chip 3 M3501 + vIN vin _

4 of 5 Experiment 11 Multistage Amplifiers Optional Experiments

4. Draw the 2 port models for the cascaded amplifier in Fig. 4. Comment on the overall gain and the loading between stages.

4.0 Optional Experiments

4.1 Common Collector as a Buffering Stage 1. Leaving the cascode and common emitter circuit intact, we now insert a Common Collector as an intermediate stage between the Cascode and Common Emitter. Find Ω the gain of this new circuit. For the Common Collector, set RBIAS = 100 k . For mea- suring input attenuation, the source resistance may have to be increased over the value in Fig. 3. Lower the amplitude of the input sinusoid and reduce the source resistance as necessary to avoid clipping at the output. 2. Draw the 2-port models for this Cascode-CC-CE configuration. Comment on the gain and the interstage loading.

FIGURE 5. Common Collector (Emitter Follower) Voltage Buffer (Lab Chip 4) V = 5V CC IBIAS is controlled by RBIAS between EF_BIAS (PIN 25) and VCC (PIN 28) on Lab Chip 4 Ω RC = 10k PIN 23 PIN 26 vOUT v PIN 24 (chip 3) b PIN 25 (chip 3)

vin3 IBIAS 10 µF

Ground = PIN 14

Experiment 11 Multistage Amplifiers 5 of 5