Lab 7 - ECE 420L 

Authored by Nicholas Banas,

Banasn1@unlv.nevada.edu

3/19/15 

  

This lab required the design of an audio amplifier using limited components.

The Core Amplifier 

  The First stage in our amplifier was based on a push pull configuration with negative feedback.  The design was based on the following schematic:
 
Push-Pull Amp

 
  The benefits of this amp are a very high gain that can be adjusted by varying R1, low output impedance and a high input impedance.  The drawback to this amplifier is that it has a high no load current draw and has an output that must stay around 5V.  Although the output impedance is relatively low, ~700 ohms, the circuit design requires an extremely low output impedance.

The First Design

  In order to achieve this decrease in output impedance we used a voltage follower output stage.  The benefits of the voltage follower is that it has a very high input impedance and a very low output impedance.  The major drawback is that it only has a gain ~1.  At this point our circuit looked something like this:
 
First design:


Output:

 
Power Draw:

 
  While this design achieves the desired goal of supplying an amplified signal to a low impedance output, it has a 5 watt current draw.  This is much too high for our liking.    

Improving the Design

  In this step we worked on improving the power draw of the amplifier.  The first step was to add resistors between the supply rails and the first amplifier stage.  While this greatly reduces the current that flows, it also reduces the output swing and the gain of the stage.  The output swing was not a concern as the supply voltage is 10V and the speaker is 8 ohms and can only handle 2 watts.  This means that the maximum rated voltage across the speaker (equal to our maximum output swing) is sqrt(2W*8) = 4 Volts.  The decrease in gain can be compensated for with a larger value for R1.  Lets see the results of putting a 10k ohm resistor between the 10v rail and the a 1k ohm resistor between the ground.    

Stage 1 Original Current

 
Stage 1 Modified Current

 

The current through the first stage is reduced by several orders of magnitude, a significant improvement.  Lets see the other effects:
 

 

 
There is a moderate reduction in gain.
 

 
Overall power consumption is down almost half, this is a significant improvement.   

Adding Gain Control

The volume control was easy, we simply added a potentiometer in series with R1, our feedback resister on the first amplifier stage.  A 0-50k ohm potentiometer gave a large range for the gain. 
 

 

  

  You can clearly see the gain increase as the potentiometer is varied.
 
 

 
  While the peak power draw increases with the gain, the average remains the same.

Further Power Reduction

  The last place that has significant power draw is the output stage.  While a large AC current is required to drive the speaker, we have an unwanted DC component that can be reduced to further reduce the amplifier's power draw.  This can be accomplished by lowering the DC bias on M3 in the output stage:
 
Stage 2 Original Current Draw

 
Stage 2 Modified Current Draw

 
You can see that the DC current through M3 is reduced by more than half.  As a side effect, at the highest gain settings there is minor distortion in the amplified signal.  This can be reduced with a higher bias, but was deemed acceptable for our amplifier.

We can see the overall effects on the amplifier:
 

 
The input resistance of the amplifier is 1/gm1||1/gm2+5k ~5044ohms.   The final output resitance of the amplifier is RSpeaker2||1/gm3 ~ 8||(1/.119) = 4 ohms.
  

 
The distortion at the highest gain settings is present in the voltage output as well.
 

 
Overall power draw is down to ~20% of the original design, a significant improvement.

Experimental Results

The setup for measuring our amplifier's performance involved an input signal of 330 mV at 1kHz.  This was chosen as the design output of an MP3 player is around .3V.  The yellow signal is the input, the blue is the output of the first stage and the purple is the final output over the connected 8 ohm 2 watt speaker.
Minimum Gain 

 
The minimum gain was 120mV/328mV ~ Av = .37
 
Maximum Gain

 
The maximum gain was 912mV/328mV ~ Av= 2.8
 
Playing Music

 

Possible Improvement

There is one additional improvement that may be made to decrease the power consumption at lower gain levels.  By clamping the input of the second stage to just above Vgs we can change the DC component of the output as the gain varies.  One possible clamping circuit is at follows:

Audio Amplifier with Clamped Output

 
Clamped output

 
  You can see the output is now clamped to ~10mV.  The clamping circuit isn't ideal, so there is a small distortion on the lower part of the waveform.
 

 The overall power reduction is very large for small gains and almost half for the larger gains.