To investigate the simple NMOS source follower amplifier also sometimes referred to as the common drain configuration.
As in all the ALM labs we use the following terminology when referring to the connections to the M1000 connector and configuring the hardware. The green shaded rectangles indicate connections to the M1000 analog I/O connector. The analog I/O channel pins are referred to as CA and CB. When configured to force voltage / measure current -V is added as in CA-V or when configured to force current / measure voltage -I is added as in CA-I. When a channel is configured in the high impedance mode to only measure voltage -H is added as CA-H.
Scope traces are similarly referred to by channel and voltage / current. Such as CA-V , CB-V for the voltage waveforms and CA-I , CB-I for the current waveforms.
ADALM1000 hardware module
1 - 1.5 KΩ Resistor (RL)
1 - small signal NMOS transistor (enhancement mode CD4007 or ZVN2110A M1)
The breadboard connections are shown in figure 1. The output of the channel A generator, is connected to the gate terminal of M1. The drain terminal is connected to the positive (+5V) supply. The source terminal is connected to both the 1.5 KΩ load resistor and Scope channel CB-H. The other end of the load resistor is connected to dround.
Figure 1 Source Follower
The channel A voltage generator should be configured for a 100 Hz Sine wave with 3 volt Max and 2 V Min. The channel B scope in Hi-Z mode CB-H is used to measure the voltage at the source. To measure the input to output error or offset, the CA-V - CB-V Math waveform can be displayed. To measure the input to output gain, the CB-V / CA-V Math waveform can be displayed.
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 source 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/2and 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 +4V to +2V swing example the minimum IS = 2V / 2.2KΩ or 0.91 mA to a maximum IS = 4V / 2.2KΩ or 1.82mA. 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 - 1 KΩ Resistor
1 - small signal NMOS transistor (M1ZVN2110A)
2 - small signal NMOS transistors (M2, M3CD4007)
Figure 2 Improved Source Follower
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 - 4.7 KΩ Resistor
1 - 10 KΩ Resistor
1 - small signal NMOS transistor ( M1 CD4007 or ZVN2110A)
The circuit configuration in figure 3 adds a resistor R2 to inject a test signal from AWG1 into the source (output) of M1. The input, gate of M1, is grounded.
Figure 3 Output impedance test
The channel A voltage generator should be configured for a 100 Hz Sine wave with a Min an Max value set so that the voltage swings +/- 1 V around the source of of M1, the 2.5 volts at the gate minus VGS. This injects a +/- 0.1mA (1V/10KΩ) current into M1's source. Scope input CB-H 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 (2.5 - VGS) / 1.5KΩ. We can calculate rsfrom this current as ohms. How does this rs compare to the value measured for the potentiometer? Change the value of R1from 1.5 KΩ to 3.3 KΩ and re-measure the output impedance of the circuit. How has it changed and why?
For Further Reading:
Return to ALM Lab Activity Table of Contents