Lab 6 - EE 420L 

Authored by Shada Sharif,

sharifs@unlv.nevada.edu

20 March 2015

Pre-lab work:

• This lab will utilize ZVN3306A and ZVP3306A MOSFETs. 
• Review datasheets for mosfets, and the simulations attached in zip file.
• Watch the video by Dr. Baker.

• This lab is about single stage transistor amplifiers. In the lab different types of amplifiers will be experimentally tested. The amplifiers are common source, common drain, and common gate. We will experimentally find the gain of each type as well as the input and output resistance.

• LTspice simulation of the circuits as well as the waveforms.
• Hand calculations for deriving the gain, input, and output resistance of the amplifiers.
• Scope pictures that prove the hand calculations experimentally. 

• The first experiment consisted of testing NMOS and PMOS common drain amplifier, which are also called source followers. Source followers have a gain of one and they are useful for high input impedence purposes, and low output impedence. They are just like a voltage follower as in op-amps. Therefore, source followers are great transition stage between a circuit that has a high output impedence that needs to be fed to a low input impedence circuit. The gate here is the input and the output is the source, drain is common for both the input and output.
• Below attached the LTspice simulations of the source follower mosfets, notice the DC offset was subtracted from the NMOS and PMOS output AC signal. 

• Below attached are the hand calculations, followed by the scope waves to verify the calculations. The experimental testing was done under high frequency so that the capacitors act as a short in AC and an open in DC so that the DC biasing is not changed. The gate here is at a higher potential so when using the capacitors the + side was facing the gate.
• The gm values were estimated from the datasheet as shown below

• Wave forms from the scope for the NMOS and PMOS gain, input resistance testing, and output resistance testing.

PMOS


 

• Above calculation shown shows how the real input or output resistance was calculated from the experiment, looking at the waves from the scopes if the output max is half the input then the resistance chosen or estimated is the correct input resistance, if the output is not half then we do the calculation of the voltage dividor shown above, leaving the resistance as a variable. This has been done for all the experiments below.
• In order to measure the input impedence of the source follower, we first had to derive the hand calculations of the input impedence, and from there we would plug in the numbers of resistors used to get a numeric answer. After having a numeric answer we would get a resistor that is similar to the value we assumed the input impedence equals. We create a voltage dividor by using a capacitor follower by the resistor we picked into the gate of the amplifier, and then apply a test voltage at the input of the capacitor. We measure the input signal and measure the peak value and also measure the signal at the end of the resistor we picked. If the signal we measured at the end of the resistor is half the input signal this means that the resistance we picked is almost equal to the input impedence/resistance of the amplifier which resulted in the half signal voltage dividor output.
• Vout=Rin/(Rin+R_picked)*Vin, if Rin=R_picked => Vout = R/2R*Vin => Vout=(1/2)*Vin.
• As for the output resistance measurement, it is a very similar concept as measuring the input resistance. So we create the same voltage dividor after doing some hand calculations, the only difference that needs to be done is that when testing the output resistance and adding a capacitor before/after the picked resistor so that the DC biasing is not disturbed, is that the junction at where the input was needs to be grounded. Then we use that same input signal as a test voltage at the output side. So basically whenever we test resistance we only apply a test voltage at the place we need to measure the resistance in and ground other junctions where an input signal was in orgionally. So the gates of the mosfets were grounded and the drains had the test voltage applied to them. 

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Experiment #2

• The second experiment was about testing common source amplifiers. Common source amplifiers are used for voltage amplifications. The input is at the gate and the output is at the drain while the source is common.
• Below are the LTspice simulations showing the gains of each MOSFET.

 

• Hand calculations of the PMOS and the NMOS gain, input and output resistance.

• Below attached the scope waveforms of the mosfets.

NMOS

PMOS

• In the above calculations, the input resistance estimated was the correct one since the output wave was half of the input.
• Looking at the derived gain one can see that the higher the Rsn and Rsp are the gain is lower. So increasing these resistances, decreases the gain.

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Experiment #3

• The third experiment was about testing common gate amplifiers. Common gate amplifiers are used as current buffers or voltage amplifiers, the input is at the source and the output is at the drain while the gate is common to both the input and the output.
• The LTspice simulations are attached below.

• Hand calculations shown below. As for Rsn and Rsp, it is the same as in the common source, as these resistances increase in value, this causes the gain to decrease. 

 

• Wave forms of the MOSFETS.

 

PMOS

• The calculation above for the Rout of the PMOS resistor used was 1k ohms and the output was half of the input.

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Experiment #4

• The last experiment was for testing a push-pull amplifier, which is an inverter where the output can go from rail to rail. The push pull is also a class AB amplifier.
• LTspice simulations are attached below

• Hand calculations are shown below

• The amplifier can be good at sourcing or sinking current since it is a class AB amplifier as mentioned earlier so it can push current into the load or pull the current out of the load. If the input and output are at the same potential the PMOS and NMOS are both on and current flows that is class A. If we have an input of VDD the PMOS is off and NMOS pushes current into load, and if the input is 0V the NMOS is off and the PMOS pulls current from the load, this is class B. So it is class AB since it can operate both ways.
• If the load resistance is changed to 510k ohms the gain will increase which can be seen in the gain formula above. This is also shown below in the second wave form where the output is saturating due to the high gain.
• Waveforms shown below.

Return to all pictures attached

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