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To investigate the simple NMOS source follower amplifier also sometimes referred to as the common drain configuration.
ADALM2000 Active Learning Module
1 - 2.2KΩ Resistor (RL)
1 - small signal NMOS transistor (enhancement mode CD4007 or ZVN2110A M1)
The breadboard connections are shown in the diagram below. The output of the waveform generator, W1, is connected to the gate terminal of M1. Scope input 1+ (Single Ended) is also connected to W1 output. The drain terminal is connected to the positive (Vp) supply. The source terminal is connected to both the 2.2KΩ load resistor and Scope input 2+ (Single Ended). The other end of the load resistor is connected to the negative (Vn) supply. To measure the input to output error, channel 2 of the scope can be used differentially by connecting 2+ to the gate of M1 and 2- to the source.
Figure 1 Source Follower
The waveform generator should be configured for a 1 KHz Sine wave with 4 volt amplitude and 0 offset. The Single ended input of scope channel 2 (2+) is used to measure the voltage at the source. The Scope configured with channel 1+ connected to display the AWG generator output. When measuring the input to output error, channel 2 of the scope should be connected to display 2+ and 2- differential.
Figure 2 Input, output waveforms
The incremental Gain (Vout /Vin) of the source follower should ideally be 1 but will always be slightly less than 1. The gain is generally given by the following equation:
From the equation we can see that in order to obtain a gain close to one we can either increase RL or decrease rs. We also know that rs is a function of ID and that as ID increases rs decreases. Also from the circuit we can see that ID is related to RL and that as RL increases ID decreases. These two effects work counter to each other in the simple resistive loaded emitter follower. Thus to optimize the gain of the follower we need to explore ways to either decrease rs or increase RL without effecting the other. It is important to remember that in MOS transistors ID = IS ( IG = 0 ).
where K = μnCox/2 and λ can be taken as process technology constants.
Looking at the follower in another way, because of the inherent DC shift due to the transistor's Vth, the difference between input and output should be constant over the intended swing. Due to the simple resistive load RL, the drain current ID increases and decreases as the output swings up and down. We know that ID is a (square law) function of VGS In this +2V to -2V swing example the minimum ID=2V/2.2KΩ or 0.91 mA to a maximum ID= 6V/2.2KΩ or 2.7mA. This results in a significant change in VGS. This observation leads us to the first possible improvement in the source follower.
The current mirror from activity 6M is now substituted for the source load resistor to fix the amplifier transistor source current. A current mirror will sink a more or less constant current over a wide range of voltages. This more or less constant current flowing in the transistor will result in a more or less constant VGS. Viewed another way, the very high output resistance of the current source has effectively increased RL while rs remains at a low value set by the current.
1 - 3.2KΩ Resistor (use a 1KΩ in series with a 2.2KΩ)
1 - small signal NMOS transistor (M1ZVN2110A)
2 - small signal NMOS transistors (M2, M3CD4007)
Figure 3 Improved Source Follower
Figure 4 Input vs output error for resistor and current source load
An important aspect of the source follower is to provide power or current gain. That is to say drive a lower resistance (impedance) load from a higher resistance (impedance) stage. Thus it is instructive to measure the source follower output impedance.
1 - 4.7KΩ Resistor
1 - 10KΩ Resistor
1 - small signal NMOS transistor ( M1 CD4007 or ZVN2110A)
The circuit configuration below adds a resistor R2 to inject a test signal from AWG1 into the emitter (output) of M1. The input, base of M1, is grounded.
Figure 5 Output impedance test
The waveform generator should be configured for a 1 KHz Sine wave with 2 volt amplitude with the offset set equal to minus the VGS of M1 ( approximately -V ). This injects a +/- 0.1mA (1V/10KΩ) current into M1's source. Scope input 2+ measures the change in voltage seen at the source.
Plot the measured voltage amplitude seen at the source. The nominal source current in M1 is (Vn - VGS) / 4.7KΩ or 720uA. We can calculate rsfrom this current as ohms. How does this rs compare to the from the test data? Change the value of R1 from 4.7 KΩ to 2.2 KΩ and re-measure the output impedance of the circuit. How has it changed and why?
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