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.