EE2B1, Laboratory Experiment 3

Multi-stage Amplifier Simulation

Aims

Objectives

Upon successful completion of this experiment, students should:

Before We Begin...

Before you leave the lab today, please fill out and submit ONE student feedback questionnaire each for the analogue electronics part (lectures, tutorials and labs) of this module. The questionnaire is a web based version of the paper one you have probably seen before and can be found by clicking this link.

Getting Started - IMPORTANT BIT!

If you have never used Micro-Cap on your account, you will need to run a set-up program which you will find in the same folder on the start menu. This program simply copies some essential data files into your Y: drive. After this, you can start the Micro-Cap program itself and begin the lab.

The set-up program creates a directory on your y: drive called y:/mc6demo/data and copies some essential component library files into it. It is VITAL that any circuits that you save are saved into this same directory. If they aren't, they will not produce valid results.

Introduction

Figure 1 shows the circuit diagram of the amplifier you will be simulating during this session. To do this, the structure of the session will be:

  1. Designing and testing a current mirror to provide the constant current source.
  2. "Constructing" and testing the differential input stage using the current mirror.
  3. Adding the common-emitter second stage and testing the circuit.
  4. Using negative feedback to improve the amplifier's performance.
  5. Comparison with an operational amplifier.


Figure 1. The two-stage amplifier.

1. Current Mirror

Figure 2 shows the circuit diagram for a current mirror suitable for use in the differential input stage. To simulate the circuit using Micro-Cap, you simply draw the components and wires onto the screen. The transistors can be drawn either by selecting the transistor icon from the toolbar or by selecting the correct device from the menu - Components|Analog Library|BJT|2N0000-|2N3904. The resistor can also be selected from either the toolbar or the components menu and should be given the value of 85k. V1 is a battery forming the negative power supply and can be selected from the toolbar or from Components|Analog Primitives|Waveform Sources. It should be given the value of 9. V2 is an independent voltage source, it is found in Components|Analog Primitives|Waveform Sources|V. It does not require a value and it isn't, in fact, part of the current mirror at all. We will only use it as a test signal source during the d.c. analysis. The wires connecting the components can be drawn using wire mode, selected either from the toolbar or by pressing Ctrl-W. Finally, all circuits must have a 0 V reference indicated by the two earth symbols in the diagram.


Figure 2. Micro-Cap diagram of the current mirror.

In theory, the collector current of Q1 should be a constant value of 100 mA regardless of the voltage supplied by V2. The actual relationship can be simulated using d.c. analysis:

The graph that should appear shows a plot of collector current vs. collector voltage for Q1. Comment on your results paying particular attention to any ways that the graph deviates from theory.

2. Differential Amplifier

Satisfied that the current mirror works, we can now use it to bias a differential amplifier. Figure 3 shows the circuit diagram. Use 2N3904 devices for all transistors. Note that for testing purposes, one of the amplifier inputs is connected to 0V and the other to an independent voltage source. We will be using this for a.c. and transient analysis so it does require a value this time which should be: "ac 0.001 sin 0 0.001 1000" (indicating that the a.c. analysis will use a signal of 1 mV amplitude and that the transient analysis will use a sine wave of 0 V mean, 1 mV amplitude and 1000 Hz frequency).


Figure 3. The differential amplifier.

DC Analysis

To measure the large signal input-output characteristics of the circuit, you need to know the identifying numbers that the software has given to the nodes in the circuit. These can be viewed by enabling Options|View|Node numbers. Select DC analysis as before but this time, the name of variable 1 should be V3 (or whatever you or the program has chosen to call the input source). The range of the variable should be ±500 mV (i.e. 0.5,-0.5,0.01). You should produce two plots, one for each of the two collector voltages. Enable "auto scale ranges" and you won't need to worry about the X & Y ranges of the plots.

Compare your plots with theoretical predictions.

Transient Analysis

Select transient analysis this time and set the dialogue box options to be:

Verify that inverting and non-inverting amplification is achieved and note any distortion or voltage offset visible. Comment on your plots.

AC Analysis

AC analysis is used to measure features like voltage gain as a function of frequency. Select AC analysis from the Analysis menu and run the simulator with the options:

Measure the frequency response for both outputs and comment on any differences. Note the peak gain of the amplifier and the upper cut-off frequency (-3dB point). Calculate the gain-bandwidth product of this amplifier. Comment on your results and, where appropriate, compare with theoretical predictions.

3. Two-stage Amplifier

Using a 2N3906 transistor (the PNP equivalent of the 2N3904), extend your differential amplifier by adding a second stage, this time in the form of a common-emitter amplifier. The new circuit should be of the form shown in figure 4.


Figure 4. The two-stage amplifier.

Perform the same DC, transient and AC analyses as for the single stage amplifier but this time using the collector of Q5 as the single output voltage. Comment on your results.

4. Feedback

100 % Negative Feedback - Buffer Amplifier

The simplest negative feedback network consists of a single wire giving 100% feedback, as with the buffer amplifier op-amp configuration. To do this with your circuit, disconnect 0V from the base of Q4 and connect the base to the output (Q5 collector) instead. Analyse the circuit using:

Comment on the effects of the negative feedback in terms of linearity, voltage gain and amplifier bandwidth.

Non-inverting Amplifier with Feedback

To obtain a closed-loop gain higher than unity, a potential divider must be introduced into the feedback loop. To get a theoretical gain of 20 dB (voltage ratio of 10), the circuit would appear as in figure 5.


Figure 5. Two stage amplifier with feedback.

Analyse this circuit using:

Again, comment on the effects of the negative feedback in terms of linearity, voltage gain and amplifier bandwidth.

5. Operational Amplifier Comparison

Both of the circuits in part 4 could, of course, have been constructed using an operational amplifier instead of the two-stage amplifier. To compare the performance of an op-amp, construct the circuit illustrated in figure 6.


Figure 6. Op-Amp equivalent of figure 5.

Perform the same analysis on this circuit as for the one in figure 5 and compare the results. Comment on the relative merits of both circuits.

Deliverables

1. Current mirror: Sketch of DC analysis curve and comments. 10 %
2. Differential input stage: Sketches of DC, transient and AC analyses with comments. 20 %
3. Two-stage amplifier: Sketches of DC, transient and AC analyses with comments. 20 %
4. Negative feedback: Sketches of DC, transient and AC analyses with comments and comparisons with (3). 20 %
5. Operational amplifier: Sketches of DC, transient and AC analyses with comments and comparisons with (4). 20 %
6. Conclusions: Stating what you have learned during this experiment. 10 %

Feedback Questionnaire Reminder

Before you leave the lab, make sure you have filled out and submitted ONE feedback questionnaire each. This is very important as it helps us to refine courses for subsequent years by identifying good and bad features of the course. Also, it will help me to attempt to remedy common complaints/problems for this year's course before the critical exam period. The questionnaire can be found by clicking this link.


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