Lab 6 - ECE 420L
Authored
by Silvestre Solano,
Email: Solanos3@unlv.nevada.edu
3-20-2015
This
lab will utilize the ZVN3306A
and ZVP3306A
MOSFETs.
Below are schematics
for NMOS and
PMOS source followers
amplifiers (also known as
common-drain amplifiers).
In
your lab report discuss the operation of these circuits.
The
above amplifiers are essentially the same, except of course one is
built using a PMOS while the other is built using an NMOS. Their gains,
which are calculated below, are approximately the same. As with all the
amplifiers in this lab, the transistors are biased with the DC source
VDD. Both amplifiers have a voltage divider that divides the DC inputs
by 1/3. Therefore, both transistors have a Gate Voltage of 5/3= 1.67
Volts. The C1 and C2 capacitors are there to act as an open circuit to
prevent the DC gate voltage to be shorted to ground while not causing a
voltage drop for the AC Vin.
Simulate the operation
of these
amplifiers.
| Schematic | Transient analysis | SPICE error log |
 |  |  |
In the above table, the gmn and gmp were determined from the simulations and are approximately gmn = 0.0183 and gmp = 0.0107.
Hand
calculate, and then verify your
hand calculations with experimentation and simulations, the
gains
and the input and output resistances ensuring that
your test
signals are at a high enough frequency that the caps have
negligible impedance but not so high that the gain is dropping off.
Gain (NMOS)
Gain (PMOS)
Rin (For both NMOS and PMOS)
Rout (NMOS)
Rout (PMOS)
If
you build this circuit using electrolitic capacitors, assuming
the
input AC signal swings around ground, put the "+" terminal of the cap
on the gate of the MOSFET. Please indicate, in your lab report, that
you understand why the capacitor is connected this way.
This is because the gate is at a higher DC potential than the AC input, which is essentially DC ground.
In
your lab report discuss, in your own words, how to measure the
input resistance.
To
measure the input resistance, I put a 33k Ohm resistor in between the
gate of the transistor and the 1uF capacitor. Then, I will measure the
gate voltage with respect to ground. If the input resistance is indeed
the calculated value of 33K Ohms, then the measured voltage
should be 1/2 of the input voltage. Essentially, I am creating a
voltage divider with the 33k Ohm resistor and the Rin of the input.
This works for both NMOS and PMOS By measuring the voltage across Rin,
I will be able to determine Rin as follows:
To
verify Rout, the procedure is very similar to the verification of Rin.
For Rin, I put a 51 Ohm resistor in series with the output Voutn to a
10 uF capacitor that goes to ground. The 10 uF capacitor is there to
act as an open circuit in DC in order to preserve the DC bias. Again,
this is essentially a voltage divider. If the Rout is actually 51.8
Ohms, then the ouptut voltage should be half of the input voltage. This
is shown below for the NMOS:
The calculation for PMOS Rout is the same except a 85.5 Ohm resistor is used.
Below are the results of the lab experiment.
| Vout/Vin (Gain) | Rin | Rout | |
| NMOS |  |  |  | |
| NMOS calculations | Vout/Vin = 186 mV / 208 mV = 0.89 | Vout/Vin = 98 mV / 216 mV = 0.45 | Vout/Vin = 116 mV / 208 mV = 0.56 | |
| PMOS |  |  |  | |
| PMOS calculations | Vout/Vin = 232 mV / 264 mV = 0.88 | Vout/Vin = 120 mV / 212 mV = 0.57 | Vout/Vin = 112 mV / 204 mV = 0.55 | |
As
seen in the above table, the experimental results suprisingly seem to
match the hand calculations and simulations. Both the gain of the NMOS
and PMOS amplifiers are both essentially 1 as expected and the
experimental results for both Rin and Rout of both NMOS and PMOS are
all very close to 1/2.
Below
are two common-source amplifiers.
Discuss
the operation of these amplifiers in your lab report including both DC and
AC operation.
The
operation of these amplifier is essentially the same as the common
drain amplifier except that they have a higher gain due to the Rsn and
Rsp resistors. Again, both transistors are biased with DC sources while
they are analyzed using thier AC inputs. The capacitors are present to
act as open circuits in DC and prevent messing up the DC biasing. Thier
source is common to both inputs.
Hand
calculate the gains and the input/output resistances.
Gain (NMOS)
Gain (PMOS)
Rin (NMOS and PMOS)
Rout (NMOS)
According
to professor Baker, the resistance is so large for the transistor that
the Rout is "for all intents and purposes" is only the resistance of R8
which is 1k Ohms. I assumed the same is true for the PMOS.
How
does the source resistance, Rsn or Rsp, influence the gain.
Rsn
and Rsp are very small compared to the 1k Ohm resistor in parallel with
them. Thus, this makes the overall parallel resistance less than 100
Ohms, which makes the gain for both NMOS and PMOS amplifiers to be
large due to the inverse relationship of the gain and Rsp/Rsn in
parallel with the 1k Ohm resistor.
Again
compare your hand calculations to simulation and experimental results.
Simulation
| Schematic | Transient Analysis | Spice error log |
 |  |  |
According
to the above table, the gmn and gmp remain unchanged from the Common
Drain amplifier. The simulations also seem to match the hand
calculations for the gain as can be seen when comparing the output with
the input, there is an approximate gain of 5 for the PMOS and a gain of
7 for the NMOS.
Experimental Results
| Vout/Vin (Gain) | Rin | Rout |
| NMOS |  |  |  |
| NMOS calculations | Vout/Vin = 1.16 V / 0.204 V = 5.69 | Vout/Vin = 296 mV / 560 mV = 0.53 | Vout/Vin = 280 mV / 580 mV = 0.48 |
| PMOS |  |  |  |
| PMOS calculations | Vout/Vin = 536 mV / 200 mV = 2.68 | Vout/Vin = 536 mV / 200 mV = 2.68 | Vout/Vin = 144 mV / 336 mV = 0.43 |
The
procedure for how to determine Rout and Rin from measurements in lab is
identical to the Common Drain amplifier previously discused. So, if the
ratio of input to output is about 0.5, then the Rin and Rout closely
match the hand calculations. For the NMOS gain, the experimental value
and calculated value are off. They have about a 17 percent difference,
but I thing it is okay. However, the gain for the PMOS is about half of
what it should have been. These descrepencies are probably caused by
slightly different gmn and gmp from the ones determined in the
simulation.
Below
are two common-gate amplifiers.
Discuss
the operation of this amplifier in your lab report including both DC
and AC operation.
The
operation is the same as the Common Source amplifier except that the
Vin is now on the Rsn and Rsp while the gates for both transistors are
grounded. Because the gate is common to the inputs and outputs, this
amplifier is called a Common Gate amplifier. Again, the DC component is
there to bias the transistors and the capacitors are there in order to
preserve correct DC biasing conditions such as preventing the DC gate
voltage from being shorted to ground. The AC component is what is
measured and analyzed in lab.
Hand
calculate the gains and the input/output resistances.
In
order to simplify the calculations, I will make the simplification that
most of the current id will go through the Rsn or Rsp instead of the 1k
Ohm resistor.
Gain (NMOS)
Gain (PMOS)
Rin (NMOS)
Rin (PMOS)
Rout (NMOS and PMOS)
Again, professor Baker said it was about 1K Ohms
How
does the source resistance, Rsn or Rsp, influence the gain.
Again,
the gain is inversely proportional to Rsn or Rsp, which would cause the
gain to increase as Rsn or Rsp decrease. In this circuit, Rsn and Rsp
are both small compared to the other resistors used.
Again
compare your hand calculations to simulation and experimental results.
Simulation
| Schematic | Transient analysis | Spice error log |
 |  |  |
Experimental resuls
| Vout/Vin (Gain) | Rin | Rout (Input) | Rout (Output) |
| NMOS |  |  |  |  |
| NMOS calculations | Vout/Vin = 1.24 V / 0.280 V = 4.43 | Vout/Vin = 152 mV / 304 mV = 0.5 | Vout/Vin
= 304 mV / 420 mV = 0.72 (note that Vin is the output without the 152
Ohm resistor and Vout is the output with the 152 Ohm resistor.) |
| PMOS |  | |  |  |
| PMOS calculations | Vout/Vin = 680 mV / 272 mV = 2.5 | Vout/Vin = 152 mV / 304 mV = 0.5 | Vout/Vin = 480 mV / 900 mV = 0.53 (note that Vin is the output without the 185 Ohm resistor and Vout is the output with the 185 Ohm resistor.) |
For
the NMOS amplifier, the gain is about 4.43, which is significantly
lower than the calculated and simulated value of 6.47. For the PMOS
amplifier, the gain is about 2.5, which is almost exactly half of what
was calculated and simulated. As before, the PMOS is about half of its
expected value and the NMOS gain is off. This could be due to a
different number of reasons ranging from human error to different gm
paramaters. I believe that it is likely the gm parameters that are
different than those of the simulations. For the Rin and Rout, if the
ratio of input to output is about 0.5, then the calculated value of the
Rin and Routs are pretty accurate. For the Rout of both the NMOS and
PMOS, I had to take into account that the gain was large from the Vout,
and I had to measure the Vout when no resistor was attached (either the
152 or 185 Ohm resistor) in order to get and accurate input reading and
then I added the resistor( either the 152 or 185 Ohm resistor) in order
to get the output. Ther ratio of these two had to be close to 0.5 using
the voltage divider technique previously described. As seen in the
above table, the ratios are about 0.5. The NMOS is a little off and the
PMOS is very closes. In my opinion, this strongly suggests that the
calcluated values of Rin and Rout of both amplifiers are accurate.
Below
is a push-pull amplifier.
Discuss
the operation of this amplifier in your lab report including both DC
and AC operation.
At
a first glance, this amplifier is clearly an inverter with a 100k
resistor connecting the input and outputs. It is this resistor that
greatly influences the AC gain of this amplifier. Apparently, if
the R1 resistor is increased, the gain will also be increased. In other
words, the gain is directly proportional to the value of the R1
resistor.
Hand
calculate the gain of this amplifier.
=3940
Do
you expect this amplifier to be good at sourcing/sinking current? Why
or why not?
No,
I do not believe it is good at sourcing or sinking current because of
the large 100k restor which greatly limits the current that can be
achieved.
What
happens to the gain if the 100k resistor is replaced with a 510k
resistor? Why?
The
gain increases as is shown in the experimental results. However, I
don't believe this was supposed to happen. What should have happened is
that the 510k Ohm was large enough that the output would be saturated
due to the ridiculously high gain. In other words, the output would
swing to AC ground since the source of both transistors are at AC
ground. The 510k Ohm resistor would almost be like an open circuit and
the amplifier would act as a simple inverter, but for the AC analysis,
the output would swing to ground.
Again
compare your hand calculations to simulation and experimental results.
Note
that the gain of this amplifier is large so the output may saturate at
VDD and Ground. To avoid this saturation you can reduce the AC input
voltage using a voltage divider.
Simulation
| Schematic | Transient analysis | Spice error log |
 |  |  |
Experimental Results
| 100k Ohm | 510k Ohm |
| Gain |  |  |
| Gain calculation | Vout/Vin = 472 mV / 10 mV = 47.2 | Vout/Vin = 1.20 V / 10 mV = 120 |
In
the lab experiment, I used a voltage divider using a 1k Ohm resistor in
series with a 9k Ohm resistor in order to divide the input voltage by
10. The input shown in the above scope pictures is the undivided input.
Howver the gain is considerably less than what was calculated and
simulated. I expected a gain of 4000, but the gain is clearly not even
one tenth. I originally used a 1 mV input to get a 4 volt output, but
that did not seem to work at all, so I tried the 10 mV input. It is
possible that for the voltage divider the resistors were to large for a
1 mV output, which would explain the amplifiers failure to work
properly. However, adding the 510 k Ohm resistor increased the gain
which was unexpected as explained previously. It is likely that the gm
parameters are much, much smaller than the simulated values of gmn =
18.8m and gmp = 20.6m.
As always, I will back up my stuff as shown below.
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