“The source load resistor can now be replaced with the current mirror from the previous Student Zone experiment to keep the source current of the amplifier transistor fixed. A current mirror can obtain a relatively constant current over a wide voltage range. This relatively constant current in the transistor results in a fairly constant VGS. From another perspective, the extremely high output resistance in the current mirror can effectively increase RL, but rs remains at the low value set for the current.
“
By: Doug Mercer, Consulting Researcher, Analog Devices; Antoniu Miclaus, Systems Applications Engineer
Target
The purpose of this experiment is to study a simple NMOS source follower, sometimes referred to as a common drain configuration.
Material
► ADALM2000 Active Learning Module
► Solderless Breadboard
► Jumper
► One 2.2 kΩ resistor (RL)
► One small signal NMOS transistor (M1 uses enhancement mode CD4007 or ZVN2110A)
illustrate
The breadboard connections are shown in Figures 1 and 2. The output of the waveform generator W1 is connected to the gate terminal of M1. Oscilloscope input 1+ (single-ended) is also connected to the W1 output. The drain terminal is connected to the positive (Vp) power supply. The source terminal is connected to the 2.2 kΩ load resistor and the oscilloscope input 2+ (single-ended). The other end of the load resistor is connected to the negative (Vn) power supply. To measure the input-output error, connect 2+ to the gate of M1 and 2C to the source to Display the difference for channel 2 of the oscilloscope.
Figure 1. Source Follower
hardware setup
The waveform generator is configured as a 1 kHz sine wave with a peak-to-peak amplitude of 2 V and an offset of 0. The single-ended input (2+) of channel 2 of the oscilloscope is used to measure the voltage at the source. The oscilloscope is configured to connect channel 1+ to display the AWG generator output. When measuring input-output error, channel 2 of the oscilloscope should be connected to display the difference between 2+ and 2C.
Figure 2. Source Follower Breadboard Circuit
procedural steps
Configure the oscilloscope to capture multiple cycles of the two signals being measured. The resulting waveform is shown in Figure 3.
Figure 3. Input and output waveforms of a source follower
The gain of the source follower (VOUT/VIN) is ideally 1, but is always slightly less than 1. The gain is calculated by Equation 1 below:
As you can see from the formula, to get a gain close to 1, we can increase RL or decrease rs. It can also be seen that rs is a function of ID, as ID increases, rs decreases. Also, as can be seen from the circuit, ID is related to RL, and if RL increases, ID decreases. In a simple resistively loaded emitter follower, these two effects cancel each other out. So, to optimize the gain of the follower, we need to find a way to reduce rs or increase RL without affecting the other side. Note that in MOS transistors, ID = Is (IG = 0).
Among them, K = μnCox/2, λ can be considered as a constant related to the process technology.
From another point of view, the difference between input and output should be constant over the expected swing because of the DC offset of the transistor Vth itself. Influenced by a simple resistive load, RL, the leakage current ID rises and falls as the output swings up and down. We know that ID is a function of VGS (square relation). Taking a +1 V to -1 V swing as an example, minimum ID = 1 V/2.2 kΩ or 0.45 mA and maximum ID = 6 V/2.2 kΩ or 2.7 mA. Therefore, VGS will change significantly. Based on these experimental results, we can improve the source follower in one aspect.
The source load resistor can now be replaced with the current mirror from the previous Student Zone experiment to keep the source current of the amplifier transistor fixed. A current mirror can obtain a relatively constant current over a wide voltage range. This relatively constant current in the transistor results in a fairly constant VGS. From another perspective, the extremely high output resistance in the current mirror can effectively increase RL, but rs remains at the low value set for the current.
Enhanced source follower
Additional material
► A 3.2 kΩ resistor (1 kΩ and 2.2 kΩ resistors in series)
► A small signal NMOS transistor (M1 uses ZVN2110A)
► Two small signal NMOS transistors (M2 and M3 use CD4007)
illustrate
The breadboard connections are shown in Figures 4 and 5.
Figure 4. Enhanced source follower.
hardware setup
The waveform generator is configured as a 1 kHz sine wave with a peak-to-peak amplitude of 2 V and an offset of 0. The single-ended input (2+) of channel 2 of the oscilloscope is used to measure the voltage at the source. The oscilloscope is configured to connect channel 1+ to display the AWG generator output. When measuring input-output error, channel 2 of the oscilloscope should be connected to display the difference between 2+ and 2C.
Figure 5. Enhanced Source Follower Breadboard Circuit
procedural steps
Configure the oscilloscope to capture multiple cycles of the two signals being measured. The resulting waveform is shown in Figure 6.
Figure 6. Enhanced Source Follower Waveform
Source follower output impedance
Target
An important aspect of a source follower is providing power or current gain, i.e. driving a low resistance (impedance) load from a high resistance (impedance) stage. Therefore, measuring the output impedance of the source follower is instructive.
Material
► A 4.7 kΩ resistor
► A 10 kΩ resistor
► A small signal NMOS transistor (M1 uses CD4007 or ZVN2110A)
illustrate
The circuit configurations in Figures 7 and 8 add a resistor R2 to inject the test signal from AWG1 into the emitter (output) of M1. The input (the base of M1) is grounded.
Figure 7. Output Impedance Test
Figure 8. Output Impedance Test Breadboard Circuit
hardware setup
The waveform generator is configured as a 1 kHz sine wave with a peak-to-peak amplitude of 2 V and an offset of VGS minus M1 (approximately CV). This injects ±0.1 mA (1 V/10 kΩ) into the source of M1. Oscilloscope input 2+ measures the change in source voltage.
procedural steps
Plot the magnitude of the voltage measured at the source. Configure the oscilloscope to capture multiple cycles of the two signals being measured. The resulting waveform is shown in Figure 9.
Figure 9. Output Impedance Test Waveform
question:
Can you briefly describe two ways to increase the source follower gain (closer to 1)?
You can find answers to your questions on the Student Zone blog.
About the Author
Doug Mercer graduated from Rensselaer Polytechnic Institute (RPI) in 1977 with a bachelor’s degree in electrical engineering. Since joining Analog Devices in 1977, he has contributed directly or indirectly to more than 30 data converter products and holds 13 patents. He was named an ADI Fellow in 1995. In 2009, he transitioned from full-time employment and continued as an honorary researcher as an ADI consultant, writing for the Active Learning Initiative. In 2016, he was appointed as an engineer-in-residence of the RPI ECSE department. Contact information:[email protected]
