EE 501 Lab 10 Output Amplifier Due: December 10th, 2015 Objective: Get familiar with output amplifier. Design an output amplifier driving small resistor load. Design an output amplifier driving large capacitive load. Simulate Monticelli s (whose paper can be found on the class and lab webpage) rail to rail push pull output stage. Introduction: The primary objective of an output amplifier is to efficiently drive signals into an output load. The output load may consist of a resistor, or a capacitor, or both. In general, the output resistor will be small, in the range of 50-100Ohms, and the output capacitor will be large, in the range of 5-100pF. The output amplifier should be capable of providing sufficient output signal (voltage, current, or power) into these types of loads. Pre-lab: A. In nature, what is the kind of the amplifier used in lab 6 (a voltage, current or transconductance amplifier)? Explain why. B. What are the requirements of driving small output resistance load (large load current)? C. Are there the same requirements of driving large capacitor load as in B? Tasks: Example circuits can be found on the webpage. Circuits designed by yourself are preferred. A. Source follower 1. Simulate loop gain frequency response of the given amplifier. Record the DC loop gain, phase margin and GB. 2. Add a 200-Ohm resistor as the load at the output node and re-simulate the loop gain frequency response and record the DC loop gain, phase margin and GB. Explain the differences with A.1. 3. Add a source follower output stage to buffer and driving the resistor as shown In Fig. 1. The common gate transistor should be large enough to handle the large current.
Fig. 1 source follower Note: When a source follower is used as the output amplifier, the original output stage is usually redesigned with much smaller current. Instead, the source follower will carry the maximum current to improve the driving capability. In this design, with regard to the limited time, you do not need to change the original circuit but just add a source follower to buffer it. 4. With the simple source follower, DC gain can be maintained to some extent with large current load. However, a supper source follower can be used to further improve the DC gain performance. It is shown in Fig. 2. Try to derive the output impedance expression and compare it with A.3. Fig. 2 super source follower 5. Change the load resistor s value to 50 Ohms and observe the variation in A.3 and A.4 s specifications. 6. Test-bench are shown below,
Fig. 3 Test-bench for A B. When the amplifier is used to drive large capacitor load, push-pull output amplifier can be used (the load capacitor is 20pF). The circuit is shown in Fig. 4. It is from Monticelli whose paper can be downloaded from the class and lab webpage. Fig. 4 An amplifier with push-pull output stage The biasing strategy can be found in Fig. 5
Fig. 5 biasing strategy To assure self-robust, the current through two floating transistors P18 and N22, and the current through corresponding bias path need to be matched. Also, the output push pull transistors and certain biasing transistors in circles need to be matched. This will assure equal current source and sink ability for the output stage. 1. An example circuit is given with the original sizing values. You can adjust the size and biasing circuit and explain your strategy. Point out voltage values of which points should be matched and why. 2. Use the inverter buffer configuration as the test bench to avoid exceeding ICMR as shown in Fig. 6. Generate frequency response, DC gain, GB, phase margin and 40K output swing range (be careful that the feedback coefficient is 0.4). 100K C. Simulate the output swing of A and B and draw your conclusions.
Fig. 6 Test-bench D. Find the total harmonic distortion (THD) of A and B (See instructions on webpage). Note: THD and SFDR are two specifications in projects. It is required to know how to deal with them. IF DIFFICULTY HAPPENS, ASK TA FOR TEST-BENCH.