Lab 6: Single-Stage Transistor Amplifiers ECE 420L 

Authored By: Joey Yurgelon

Email: yurgelon@unlv.nevada.edu

March 17, 2015

Pre-lab Work:

• This lab will utilize the ZVN3306A and ZVP3306A MOSFETs. 
• Review these datasheets and become familiar with these transistors.
• Verify that the simulations seen in lab6_sims.zip reasonably model the behavior of the transistors' ID v. VGS, ID v. VDS, and gm v. VGS curves.
• Finally, watch the video single_stage_amps and review single_stage_amps.pdf 

• Students will understand how to experimentally calculate the input and output resistances of single-stage transistor amplifiers.

• 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.
  
• Simulate the operation of these amplifiers.
• 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.
• 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.
  
• In your lab report discuss, in your own words, how to measure the input resistance.
• For measuring the input resistance add a resistor equal to the value you calculated between the input voltage source and the amplifier.
• Measure the peak AC current through this added resistor by taking the difference in the peak AC voltages across the resistor (on one side is the input voltage signal and the other side is the connection to the amplifier's input capacitor) and then dividing by the resistor's value.
• Measure the peak AC voltage on the input of the amplifier (the left side of the capacitor).
• Dividing this peak AC voltage by the peak AC current through the added resistor is the amplifier's input resistance.
• Again, in your lab report discuss how to measure the output resistance.
• For measuring the output resistance, add a resistor equal to the value you calculated in series with a big capacitor (to avoid messing up the biasing) from the amplifiers output to ground. 
• Measure the peak AC current through this added resistor.
• Measure the peak AC voltage (remove the DC component) on the gate of MOSFET and the peak AC voltage on the source of the MOSFET. The difference in these two AC voltages is the peak AC gate-source voltage of the MOSFET.
• Dividing this peak AC gate-source voltage by the peak AC current through the added resistor is the amplifier's output resistance.

Experimental Results: 

• Note: Due to the nature of the waveforms used to determine input/output resistance (Output halved due to divider effect), only the output resistance waveforms were shown. For the first four amplifiers, the input resistance is theoretically the same between CD and CS.

    Exercise #1: Experimentally determine the input and output resistances of the common-drain amplifiers. Common Drain (CD).

• The first set of amplifiers that needed to be built in this lab are known as Common Drain (CD). They are characterized by a gain of nearly 1x, low input capacitance, and high input impedance. For these amplifiers, the mosfets are biased by the voltage dividers infront of the gates which in combination of the resistor in the source, sets the Gate-Souce voltage at saturation level. The input is AC coupled so that the input does not disrupt the DC biasing. The output is taken from the source of the amplifier. After determining biasing, and the AC current through the mosfets, the input/output resistances are important parameters to determine. 
• To determine the input resistance, we need to refer to our hand calculations (seen below), and place the value determined between the input voltage source and the amplifier. One would then determine the AC current through this added resistor by measuring both sides, and dividing by the resistance. From there, one can simply take the peak input voltage and divide by the AC current determined. This will result in the experimental input resistance. The following formula can be determined: Rinput = (Vinp)/(ACI) where Vinp is the peak input voltage, and ACI = (Vr)/Radded or the voltage drop accross the added resistor divided by its resistance. To verify the results, place the experimentally determined input resistance between the input voltage source and the amplifier, and measure the voltage at the gate and source. One should see a voltage divider effect, and see the output halved. 
• To determine the output resistance, we need to once again take the result from our our hand calculations below, but instead of placing the resistor at the input, we will place it in series with a big capacitor from the amplifiers output to ground. We will then measure the AC voltage across the added resistor, and then divide the by added resistance to determine the peak AC current through the resistor. One will then need to determine the peak AC voltage which is determined by measuring the voltage at the gate of the mosfet as well as at the source, and then subtracting them. From here, one will simply need to divide by the peak AC voltage by the peak AC current to determine the experimental output resistance. The following formula help illustrate the point: Iacp = (Vr)/Radded, Vp = Vg - Vs, Rout = Vp/(Iacp) where Iacp is the peak AC current, Vr is the voltage across the added resistor, and Vp is the peak AC voltage. Similarly to the input resistance, one should be able to see a voltage divider effect in which the output is divided in half. 

Common Drain Hand Calculations
NMOS/PMOS Common Drain Gains	Common Drain Output Resistance
NMOS Common Drain Experimental Gain < 1x	NMOS CD Experimental Output Resistance
PMOS Common Drain Experimental Gain < 1x	PMOS CD Experimental Output Resistance

• Below are two common source amplifiers. Both are are biased by the voltage divider located at the gate as well as the resistors in the drain and the source. This will set our bias, and will be able to hold the mosfet in the saturation region. The added capacitors only come into play when looking at the AC operation of the circuit. They should be large enough to appear as shorts, and prevent any issues in the biasing when applying an AC input voltage. The common source, unlike the common drain, is characterized by the gain of the amplifier. It is typically much large than 1x. In this case, Rsn or Rsp can be changed to either increase or decrease the gain. By increasing Rsn or Rsp, we are effectively decreasing the gain in the common source amplifier. This is due to the fact that the gain, which can be seen below, contains an Rsn/Rsp term in the denominator, and thus by increasing the value in the denominator, we decrease the gain. The converse is also true.

Common Drain Hand Calculations
NMOS/PMOS Common Source Gains	Common Source Output Resistance
NMOS Common Source Experimental Gain	NMOS CS Experimental Output Resistance
PMOS Common Source Experimental Gain	PMOS CS Experimental Output Resistance

• The common gate amplifer follows similar suit as the common source. The difference, however, comes into play with the amplifiers gain which is no exhibits an 180 degree phase shift, and the input resistance is much smaller. In the common source and the common drain above, the input resistance was determined to be 33.3kOhm, but for the common gate, it is determined to be approximately 156 Ohms. As far as biasing is concerned, the common gain is DC biased the same as the previous amplifiers, and coupled with capacitors so that any AC input would not affect the biasing. 

Common Gate Hand Calculations
NMOS/PMOS Common Gate Gains	Common Gate Output Resistance
NMOS Common Gate Experimental Gain	NMOS CG Experimental Output Resistance
PMOS Common Gate Experimental Gain	PMOS CG Experimental Output Resistance

• The push-pull amplifier is biased by the 5 V source voltage. There is no current through the 100kOhm resistor to the gates which sets the gate at a voltage to keep both of the mosfets in saturation. The AC voltage input is coupled by a capacitor to prevent any messing up of the bias. From the hand calculations below, we can see that we are able to look at the output resistance of each mosfet, and add them together to characterize the input/output relationship of the amplifier where the resistor acts as a gain multiplier. By this result we can determine that if we were to change this resistor to a much higher value, such as 510kOhm, out gain would increase. This amplifer is good at both sourcing/sinking current due to the topology at the output. The PMOS has the ability to source current when on, and the NMOS can sink current when the on. 

Push-Pull Hand Calculations
Push-Pull Spice Schematic	Push-Pull Output Resistance
Push-Pull with 100kOhm	Push-Pull with 510kOhm

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