Lab

Lab 7 - ECE 420L

Authored by Stephanie Silic

silics@unlv.nevada.edu

March 29th, 2017

Lab Description

Design of an audio amplifier

This lab will once more utilize the ZVN3306A and ZVP3306A MOSFETs.

Design an audio amplifier (frequency range from roughly 100Hz to 20kHz ) assuming that you can use as many resistors, ZVN3306A transistors, and ZVP3306A transistors as you need along with only one 10uF capacitor and one 100uF capacitor. Assume that the supply voltage is 10V, the input is an audio signal from an MP3 player (so your amplifier should have at least a few kiloohms input resistance), and the output of your design is connected to an 8 Ohm speaker (so ideally the output resistance of your amplifier is less than 1 Ohm).

Lab report should include:

thoughts on the design of the amplifier including hand calculations. (A good place to start is with the push-pull amplifier characterized in lab 6.)
Simulation of design
Build and testing results of design.
performance of the design including power dissipation, output swing, input resistance, output resistance.

Lab Report

An audio amplifier must take a small signal from music (say an mp3 plalyer), which has such a small magnitude that it is not audible to the human ear, and amplify the signal to something we can hear through a speaker.

For this design process, I begin by simulating the single push-pull amplifier to understand what's going on. The circuit used was given in the prelab and can be found in lab7_sims.zip.

Vout1 shows what happens when the signal is directly connected to the 8 ohm speaker. Most of the voltage is dropped accross the much bigger 10k source resistance, so the output is barely detectable.

Vout2 shows what happens when the signal is amplified by a push-pull amplifier. The gain of the push-pull amplifier with no load attached was calculated in lab 6 as follows:

In the amplifier simulated above, the value of R1 is 10k, which would mean the push-pull amplifier would have a gain of about 280 under no load.

As noted in the design specifications, the output resistance of the amplifier should be much less than 8 ohms. This is so that most of the output voltage is dropped across the output (the 8 ohm speaker) rather than the output resistance of the amplifier.

The calculation for the input resistance of the push-pull amplifier is as follows:

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Lab7/calc_input_resistance.PNG

How can we reduce the output resistance?

Previoius students have done this by noticing that the Common Drain amplifier, or a source follower, has an output resistance much smaller than other amplifier topologies. Since the output resistance is calculated as follows (once again, from lab 6)

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Lab6/calcs_AC_nmos_CD.PNG

The circuit would be the following:

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Lab7/sch_amplifier2.PNG

Simulating the Rout of the circuit shown above, varying R2:

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Lab7/sim_Rout_parametric_cascaded.PNG

With a smaller resistor, the output resistance goes down. Ideally, this resistance is nearly 1 for an 8 ohm speaker so that enough voltage drops across the speaker.

Building and testing this circuit with a 3.3 ohm R2 value, the smalleset we have available, the gain was high enough to hear a moderate volume from the speaker. The following screenshot shows the operation of the circuit at 1kHz, with the speaker attached:

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Lab7/initial_run_with_sourcefollower.PNG

In this lab, however, we were making use of a 25 ohm speaker. Because of this, the push-pull amplifier by itself was able to work better than trying to use the source follower to decrease output resistance.

Here is the circuit and simulation of the push-pull amplifier with a 25 ohm load for the speaker, and a 20k feedback resistor for higher gain:

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Lab7/sch_25ohm_speaker.PNG

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Lab7/sim_25ohm_speaker.PNG

The power consumed by the circuit is found by multiplying the input voltage by the total current drawn by the circuit, or the current through the VDD source. For this circuit, the power drawn turns out to be about 2.3 Watts, which is acceptable for the speaker.

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Lab7/sim_power_25ohm.PNG

The Rin and Rout values are simulated as well:

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Lab7/sim_Rin_vanilla_push-pull.PNG

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Lab7/sim_Rout_vanilla_push-pull_vdd10.PNG

Since we want a higher input resistance, a parametric simulation varying R1 was done to see the change in input resistance:

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Lab7/sim_Rin_parametric_vanilla.PNG

Initially, a 10k ohm input resistance was selected, but after testing this circuit, I used a higher resistor to increase the gain.Since a high input resistance is desired, this was deemed acceptable.

Building and testing the circuit, the following oscilloscope image shows the output swing of the amplifier:

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Lab7/gain_of1_3.PNG

Oscilloscope screenshot while playing music:

The power drawn by the circuit is the current drawn multiplied by the input voltage. The power consumption is about 0.06Ax9.2V .5 Watts as shown here. The current however, did stay mainly at about 100mA, rather 60mA as shown in the picture of the power supply, which in that case would be 9.2Vx0.11A = 1.01 Watts.

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