What can you deduce about the stability of your amplifier

Question

Experiment- Design of an Operational Amplifier Using PSice

1. Learning outcomes

At the end of this lab, you will:

• be able to produce a functioning operational amplifier circuit by combining different stages.
• be aware of problems and challenges to meet a specification for designing and characterising an operational amplifier.

2. Objectives

It is required to design an operational amplifier with the aid of PSpice software to satisfy the following design specifications:

a) Differential input impedance greater than 100 kΩ.
b) Voltage gain (that is, 'open loop gain') greater than 500,000.
c) Output impedance less than 1 kΩ.
d) Output voltage to be approximately zero volts for zero input.
e) Frequency response down to DC (0 Hz).
f) Supply voltage +9 to -9 volts.
g) Total current consumption not greater than 5 mA.

3. Experimental work

3.1 Part I: Transistor output characteristics

Objective: To obtain a set of output characteristics for the two transistor types to be used in this experiment.

Obtain a plot of the output characteristics of the 2N222 transistor, i.e. I_C vs. V_CE (0 to 20 V in steps of 0.5 V) for a range of I_B (0 to 40 μA in steps of 4 μA).

Procedure:

Task-1: Input the circuit schematic of Figure 3 in PSpice schematics.

Task-2: In the 'Analysis' menu, select 'Setup.: and check the 'DC sweep' option and input the variable as V_CE^' with the desired range. Click on 'Nested sweep' option to set the base current I_B steps. Thus, for each value of I_B (0, 4, 8 .... 40 μA), V_CE is swept from 0 to 20 V.

Task-3: Close the 'Analysis setup' window. In the 'Analysis' menu, select 'Simulate' (you can do that by pressing on F11 key as well). A response curve will appear. Take a screenshot of this response.

Task-4: From the curve, estimate the DC current gain, β (also known as h_FE) at I_C˜2 mA. Estimate also the AC (small signal) current gain, β₀ (also known as h_fe). Hint: see your notes on how to do this. Record your results.

Figure 3. Schematic of Part I.

3.2 Part II: Achieving the specification of the operational amplifier Objective: To build the complete operational amplifier circuit and obtain the required specification.

Start now building the complete circuit of the operational amplifier (see the given `tutorial' lecture for ELEC271 on VITAL).

Procedure:

• Task-1: Combine the required stages to build a complete operational amplifier in PSpice. Hint: Match the differential amplifier and common emitter stages with an emitter follower stage, make the R_in^EF ˜ 10R_out^DA.

• Task-2: Obtain the transfer characteristics of your amplifier by grounding one input and performing a DC sweep from -9V to +9V on the other input. Identify the useful operating range from your plot. You will need to narrow the sweep range to achieve an accurate useful range.

• Task-3: Find the open loop gain (A_0l) of the amplifier from the useful range in the above step.

• Task-4: Determine the required DC voltage offset from the transfer characteristic. Use this value to help balance the amplifier - you can apply a small DC offset to one of the inputs to try to centre the output close to zero volts. Hint: Alternatively, you could use the technique mentioned in the lecture notes to obtain A_0l which would also give you the DC offset.

• Task-5: Obtain a set of input/output waveforms (in useful range) from a transient simulation, and from which, calculate the gain of the amplifier. Verify that the specification has been met. Record your results.

• Task-6: Obtain a set of input waveforms (in useful range) in an AC sweep simulation, add an input impedance trace and calculate the average value of it. Change the trace and calculate the average output impedance. Hint: The screenshots of the corresponding impedance (V//) should be taken - the impedance values can be found in the bandwidth.

• Task-7: Display the currents and voltages on the schematic and watch out the DC voltage levels and total current consumption.

3.3 Part III: Obtaining the frequency response of the designed amplifier Objective: To obtain the gain and phase Bode plots of the designed amplifier.

Now, obtain the frequency response of the designed amplifier by determining the gain and phase Bode plots.

Procedure:

• Task-1: Determine Bode plots (gain and phase) of your amplifier by replacing the VSIN input sources with VAC parts in PSpice and run an AC sweep. The parameters of the VAC part should be set to match those used in your VSIN part. Take screenshots of the Bode plots. Hint: The function DB(-) and P(-) may be used for the plots.

• Task-2: Add phase compensating capacitor (say 30 pF or any suitable value) between the collector of the common emitter stage and the base of the first emitter follower. Investigate the effect on the Bode plots. Take screenshots and record your findings.

3.4 Bonus Part: Response to common-mode signal Investigate the response of your amplifier to common-mode signals.

4. General questions

a) What can you deduce about the stability of your amplifier from the Bode plots in Part III?
b) What is the purpose of the 'Phase compensating capacitor?