Lab 7 - ECE 420L
Authored
by Silvestre Solano,
Email: Solanos3@unlv.nevada.edu
4-10-2015
Design
an
audio amplifier (frequency range from roughly 100 Hz to 20 kHz)
assuming that you can use as many resistors, ZVN3306A transistors, and
ZVP3306A transistors as you need along with only one 10 uF capacitor
and one 100 uF capacitor.
Assume that the supply voltage is 10 V, the input is an audio signal
from an MP3 player (and 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).
Your
lab report should detail your 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.
For
the push-pull amplifier seen above, the gain was found to be very low
in lab. We estimate that its gain was about 1. So, professor Baker
suggested we connect second NMOS to the ouput. With this suggestion,
the amplifier shown below was designed. For the push-pull
amplifier, I assume that the RS1 and RS2 resistors represent the output
resistance of the MP3 player and therefore, the real input (output of
the MP3) is to the left of the RS1 and RS2. So, for the modified design
shown below, I labeled the node that represents the input to be
amplified as Vin. I also removed the unamplified speaker from the
schematic.
The gain is calculated by using the equation shown below and substituting the approriate values.
The
above equation for the gain was kindly provided by professor Baker in
lab. This equation works if the we assume that Vin is much less than Vx
and the output resistance of all the transistors is very big. The
appropriate values for the resistors can be seen from the schematic.
The values for gm1 and gm2 are determined from the Spice error log,
which is shown after the schematic below. gm1 = 0.262 and gm2 = 0.240.
Using these values, the gain is calculated as follows:
This is a pretty big gain.
Simulate
your design. Document the results in your lab report.
Spice error log with gm1 gm2 and gm3.
Input and output waveforms
Gain
As
seen from the above frequency response, the input is amplified from at
least 100 Hz to 20 KHz as specified by the design criteria. The simulated gain is about 135 (42.6 dB).
This is very different from the calculated value of 22311. This could
be for a number of reasons. Most likely this is because the assumptions
made to justifiy the gain were wrong. The output resistances of the
transistors are probably not small enough that they can be
neglected.
Build
and test your design.
Document
the performance of the design including power dissipation, output
swing, input resistance, output resistance.
The circuit was bult in lab and the input and output waveforms where captured and are shown below.
Unfortunately,
the waveforms do not make any sense because the input from the
function generator was only 1 volt amplitude, but for some reason, the
oscilloscope shows the input to be a ridicolus 9.4 volts. Also, the
output waveform is about half of the input. I spent a fair amount
of time trying to figure out why this was so, but I could not determine
the cause. Also tried adding a second push-pull amplfier stage, but the
waveforms for that also did not make much sense. However, the sound
coming from the speaker was louder with the amplifier than without it.
So apparently, it does its job of amplifying sound.
The
power dissipation of this amplifier can be deduced by the power
supplied by the 10 volt source. The power supplied from the 10 volt
source can be determined by the picture shown below. Thus, the power
dissipated is 10V*0.29A = 2.90 watts. This is quite large for the
transistors used in lab because the transistors got very hot in the
actual experiment.
Since
the scope waveforms did not yeild accurate results, I will have to rely
on the simulation for determining the output swing and input/output
resistances. The output resistance is shown to be about 8 Ohms, which
is what I exptected. The input resistance is about in the range of the
expected 10k Ohms.
Output Resistance
Input Resistance
The output swings from about 420 mV and -330 mV, as shown by the simulated input and output waveforms shown in the begining of this report.
And as always I will back up my stuff as shown below.
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