Follow Us Objective
The objective of this activity is to investigate the simple NMOS source follower amplifier, also sometimes referred to as the common drain configuration.
Materials
- ADALM2000 Active Learning Module
- Solderless breadboard
- Jumper wires
- One 2.2 kΩ resistor (RL)
- One small signal NMOS transistor (enhancement mode CD4007 or ZVN2110A for M1)
Directions
The breadboard connections are shown in Figure 1 and Figure 2. The output of the waveform generator, W1, is connected to the gate terminal of M1. Scope Input 1+ (single ended) is also connected to the W1 output. The drain terminal is connected to the positive (Vp) supply. The source terminal is connected to both the 2.2 kΩ 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.
Hardware Setup
The waveform generator should be configured for a 1 kHz sine wave with 2 V amplitude peak-to-peak and 0 offset. The single-ended input of Scope Channel 2 (2+) is used to measure the voltage at the source. The scope is 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 the 2+ and 2– differential.
Procedure
Configure the oscilloscope instrument to capture several periods of the two signals being measured. A plot example is presented in Figure 3.
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 Equation 1.
From the equation we can see that in order to obtain a gain close to 1 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 affecting 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 +1 V to –1 V swing example the minimum ID = 1 V/2.2 kΩ, or 0.45 mA, and the maximum ID = 6 V/2.2 kΩ, or 2.7 mA. 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 previous StudentZone activities 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 fairly constant VGS. Viewed another way, the very high output resistance of the current mirror has effectively increased RL while rs remains at a low value set by the current.
Improved Source Follower
Additional Materials
- One 3.2 kΩ resistor (use a 1 kΩ in series with a 2.2 kΩ)
- One small signal NMOS transistor (ZVN2110A for M1)
- Two small signal NMOS transistors (CD4007 for M2 and M3)
Directions
The breadboard connections are shown in Figure 4 and Figure 5.
Hardware Setup
The waveform generator should be configured for a 1 kHz sine wave with 2 V amplitude peak-to-peak and 0 offset. The single-ended input of Scope Channel 2 (2+) is used to measure the voltage at the source. The scope is 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 the 2+ and 2– differential.
Procedure
Configure the oscilloscope instrument to capture several periods of the two signals being measured. A plot example is presented in Figure 6.
Source Follower Output Impedance
Objective
An important aspect of the source follower is to provide power or current gain—that is, to drive a lower resistance (impedance) load from a higher resistance (impedance) stage. Thus, it is instructive to measure the source follower output impedance.
Materials
- One 4.7 kΩ resistor
- One 10 kΩ resistor
- One small signal NMOS transistor (CD4007 or ZVN2110A for M1)
Directions
The circuit configuration in Figure 7 and Figure 8 adds a resistor, R2, to inject a test signal from AWG1 into the emitter (output) of M1. The input, the base of M1, is grounded.
Hardware Setup
The waveform generator should be configured for a 1 kHz sine wave with 2 V amplitude peak-to-peak with the offset set equal to minus the VGS of M1 (approximately –V). This injects a ±0.1 mA (1 V/10 kΩ) current into M1’s source. Scope Input 2+ measures the change in voltage seen at the source.
Procedure
Plot the measured voltage amplitude seen at the source. Configure the oscilloscope instrument to capture several periods of the two signals being measured. A plot example is presented in Figure 9.
Question:
Can you briefly describe two ways in which the gain of the source follower can be improved (closer to 1)?
You can find the answers at the StudentZone blog.
