Assignment Title

1.       Assignment Aims

Learn how to analyse, measure, and design op-amps.

2.        Learning Outcomes:

Knowledge and Understanding Outcomes

2.  Examine, compare, and evaluate appropriate mathematical methods and models required to analyse analogue circuitry and solve problems relating to analogue systems.

Ability Outcomes

3.  Examine and extract data for an unfamiliar problem before designing and

developing a solution, formulating an appropriate test strategy (using dc and ac analysis) and assessing the success and quality of the solution.

4.  Analyse and design circuit/systems to a required specification.

3.  Assessment Brief

Introduction:

As this is an individual (not group-based) assignment, by the end of this assignment each student is expected to perform. the tasks given below:

•    For the operational amplifier (op-amp) in Fig. 1 on the next page, perform. an approximate DC analysis to calculate the DC collector currents and voltages  in the circuit. Find the quiescent power dissipation for each transistor and the voltage gains of each stage. Also, calculate the AC characteristics of the circuit, such as, total voltage gain (dB), input and output impedance, Common Mode Rejection Ratio (CMRR) in dB, and maximum output voltage swing.

•    Using NI Multisim software, simulate the op-amp circuit in Fig. 1 and calculate the DC operating conditions and AC operating characteristics. Compare the results, and comment on any discrepancies found.

•    Make improvements to the simple op-amp of Fig. 1 (e.g., using current

mirrors, cascaded differential amplifiers, Darlington pairs, etc.) to upgrade its  performance. Simulate the new circuit(s) in NI Multisim and calculate the new and improved values of voltage gain (dB), input and output impedance, CMRR (dB), etc.

•   Write a report which includes the overall findings, analysis, and the results of activities performed above.

Procedure

Op-amp DC/AC Analysis (25%)

Fig. 1 shows a very simplified version of an integrated operational amplifier (op-amp) with only 5 transistors.

•    Perform. an approximate DC analysis (assuming β >> 1, so that the base current of every transistor can be ignored) to calculate the DC collector   currents and voltages in the circuit.

•    Find the quiescent power dissipation for each transistor and the voltage gains of each stage.

•   Calculate the AC characteristics of the circuit, such as, total voltage gain (dB), input and output impedance, CMRR in dB (Common Mode Rejection Ratio), maximum output voltage swing. Assume β = 400, and Early voltage = 200 V.

•    Ideally, an op-amp should have a very high input impedance (MΩ), low output impedance (10-100 Ω), very high voltage gain (> 100,000 = 100 dB), and a very high CMRR (> 100dB). Is this the case with this simple circuit?

Figure 1: A simple operational amplifier circuit.

Simulation (30%)

•   Simulate op-amp circuit in Fig. 1 with NI Multisim and calculate the DC

operating conditions and AC operating characteristics as done earlier with theoretical calculations.

•     Compare the theoretical and simulation results, and comment on any discrepancies found.

Design improved op-amp circuit (35%)

•    Using the knowledge gained in module lectures, design an improved op-amp circuit (e.g., use current mirrors, cascaded differential amplifiers, Darlington pairs, etc.) to upgrade the performance of op-amp circuit given in Fig.1.

•    Do not use resistors larger in value than 50 kΩ due to area limitations and prefer to use active components (easier to realise in IC technology) over passive ones.

•   Simulate the new circuit(s) in NI Multisim and calculate the new and improved values of voltage gain (dB), input and output impedance, CMRR (dB), etc.

Report Writing (10%)

Write a report which includes the overall findings, analysis, and the results of activities performed above.

4.        Marking Scheme