← Analog & Mixed-Signal Interviews
Interview Question · Concept
The Source Follower
The Question
The interviewer asks
What does a source follower do, why is its gain below one, and when would you use it?
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Reveal the Model Answer
It takes the input at the gate and the output at the source, so the output follows the input one V_GS lower. Its gain is g_m/(g_m + g_mb + g_ds), 0.841 on the lesson's bench. With the body at ground, the body effect causes most of the loss. Its output resistance is about 1/(g_m + g_mb), a few kilohms, so it drives loads a gain stage cannot.
- The sink fixes the current
IB pulls 20 µA out of the source node, and the only way in is through M1. So M1 carries 20 µA whatever the input does.
ID = 20 µA
- The current fixes VGS
A saturated transistor carries 20 µA at one gate-source voltage. Here VGS = 0.822 V, so the source settles 0.822 V below the gate.
- The output follows
Raise the gate by 1 mV and the source rises by 0.84 mV. VGS barely changes, so the current stays at 20 µA.
Read the circuit as text
- M1 NMOS transistor (W/L = 4/0.5): gate →
in, drain →VDD, source →out - VIN voltage source (1.2 V + v_in): + →
in, − →0 - IB current source (20 µA): + →
out, − →0 - out port: terminal →
out
in= 1.200 Vout= 0.378 V
The schematic draws three terminals. M1's body is the shared p-type substrate, at ground: in the netlist it is the fourth node, 0.
- v(in)
- v(out)
- Input voltage 1.2 Vv(in) = 1.2 V · v(out) = 377.6 mV
With 1.2 V in, the output is 377.6 mV: 822 mV lower. The ADC reads a reference less than a third of the intended value.
- Input voltage 1.6 Vv(in) = 1.6 V · v(out) = 717.2 mV
With 1.6 V in, the output is 717 mV: the gap has grown from 822 to 883 mV. The output rises more slowly than the input, and its slope is the gain.
Read the plot as a table
| Input voltage | v(in) | v(out) |
|---|---|---|
| 900 mV | 900 mV | 127.2 mV |
| 1 V | 1 V | 210.2 mV |
| 1.1 V | 1.1 V | 293.7 mV |
| 1.2 V | 1.2 V | 377.6 mV |
| 1.3 V | 1.3 V | 461.9 mV |
| 1.4 V | 1.4 V | 546.7 mV |
| 1.5 V | 1.5 V | 631.8 mV |
| 1.6 V | 1.6 V | 717.2 mV |
| 1.7 V | 1.7 V | 803 mV |
| 1.8 V | 1.8 V | 889.1 mV |
What the lesson's follower delivers: the output sits one VGS below the input and rises a little more slowly.
Self-Check
Tick each key point you covered in your answer.
What Interviewers Listen For (Fresher)
A good fresher answer says the output follows the input one V_GS lower, with a gain just below one. It adds that the output resistance is low.
What Interviewers Listen For (Mid-Level)
A good mid-level answer blames the body effect for the lost gain. It avoids it with a PMOS follower whose well is tied to its source. It checks the input range against V_GS plus the sink's V_Dsat.
Cross-Questions
Where the interviewer goes next. Open a node to practise that follow-up.
WhyWhat IfProve ItBreak ItCompare
- The Source Follower
-
WhyHow the Body Effect Costs a Follower GainL1 JuniorL2 Mid-Level
Why does the body effect reduce a source follower's gain, and how do you avoid it?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
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Reveal the Model Answer
When the source rises above the grounded body, the threshold rises too. So part of each input step goes into the extra threshold instead of moving the output. In the small-signal model that is g_mb working against g_m. A PMOS follower in its own n-well, with the well tied to its source, has no source-to-body voltage and avoids it.
Self-Check
Tick each key point you covered in your answer.
-
Prove ItSolve E3.5.1: A Bench for the Source FollowerSimulator-graded
Show it, don't just say it: this Silicon Lab problem is graded by the simulator.
-
What IfA Source Follower Driving a Heavy LoadL1 JuniorL2 Mid-Level
What if the follower drives a load resistor of a few kilohms? What happens to the gain?
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Reveal the Model Answer
The follower's own output resistance, about 1/(g_m + g_mb), forms a divider with the load. At 20 µA that is a few kilohms, so a load of similar size roughly halves the gain. More bias current raises g_m and lowers the output resistance. So a follower suits light loads, and heavy loads need a stronger output stage.
Self-Check
Tick each key point you covered in your answer.
Misconception
"A follower has unity gain, so any load is fine."
The gain is already below 1 from the body effect. A load comparable to 1/(g_m + g_mb) divides it further.
Source Lesson E3.5 key points
-
CompareSource Follower vs Common GateL1 JuniorL2 Mid-Level
Compare a source follower with a common-gate stage.
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
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Reveal the Model Answer
A follower is a voltage buffer: high input resistance at the gate, low output resistance at the source, gain just under one. A common-gate stage is a current buffer. Its input resistance at the source is low, about 1/(g_m + g_mb) while the load is small. Every signal current that enters leaves at the drain. That is why the common-gate stage is the top half of a cascode.
A Common-Gate Stage on Its BenchThe gate sits at a fixed 1.2 V. The input current enters at the source; the output is the drain. Whole circuit- The bias
IB pulls 20 µA out of the source and IL pushes 20 µA into the drain, so M1 carries 20 µA and RL carries none. The source settles at 0.372 V, one VGS below the gate.
20 µA
- A signal current arrives
An extra current iin leaves the source through IB. The source must fall a little to raise VGS and let M1 carry it.
iin
- It leaves at the drain
IL is fixed, so the extra current in M1 must come from RL. The same iin flows through the load: the stage passes the current on.
iin
Read the circuit as text
- IL current source (20 µA): + →
VDD, − →out - M1 NMOS transistor (W/L = 4/0.5): gate →
n1, drain →out, source →s - RL resistor (20 kΩ): pin 1 →
out, pin 2 →n2 - VX voltage source (0.9 V): + →
n2, − →0 - VG voltage source (1.2 V): + →
n1, − →0 - IB current source (20 µA + i_in): + →
s, − →0 - out port: terminal →
out
s= 0.372 Vout= 0.900 V
M1's body is at ground (the fourth node,
0, in the netlist). VX holds the far end of RL at 0.9 V, so the drain sits at 0.9 V for every RL.Self-Check
Tick each key point you covered in your answer.
- The bias
-
Break ItThe Input Common-Mode RangeL1 JuniorL2 Mid-Level
Asked if they ignore the input range
What if the input common-mode voltage is too low or too high for the pair?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
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Reveal the Model Answer
Too low, and the tail device leaves saturation, so the tail current falls. Too high, and an input device leaves saturation. The lesson's OTA ran from 0.811 V to 1.274 V. DC gain and follower error can hide the lower edge, so check the saturation margins and the tail current directly.
DC sweepOpen-Loop Gain and Tail Current Across the Input RangeGain from the two-follower bench (above) and M5's drain current on a log axis (below), with the measured saturation edges. Whole plot- open-loop gain
- tail current
- Input V_CM 300 mVopen-loop gain = 40.34 dB · tail current = 5.149 nA
At 0.3 V the tail carries 5.1 nA, yet the DC gain still reads 40.3 dB and a follower would be only 6.3 mV off. Nothing at DC warns you.
- Input V_CM 810.6 mVopen-loop gain = 42.85 dB · tail current = 17.44 µA
M5 leaves saturation at 0.811 V; the hand estimate was 0.830 V. Below this the tail current falls by decades.
- Input V_CM 900 mVopen-loop gain = 42.73 dB · tail current = 18.67 µA
Mid-range: 42.73 dB and 18.67 µA.
- Input V_CM 1.274 Vopen-loop gain = 41.26 dB · tail current = 19.66 µA
M1 leaves saturation at 1.274 V; the hand estimate was 1.255 V. Body effect has raised Vth1 from 0.637 V to 0.702 V here, because the tail rose with the inputs while M1's body stayed at ground.
- Input V_CM 1.4 Vopen-loop gain = 39.43 dB · tail current = 19.77 µA
Above the edge the gain sags to 39.4 dB and the follower's error grows to 60.9 mV.
Read the plot as a table
Input V_CM open-loop gain tail current 0 V 18.85 dB 4.168 pA 200 mV 38.43 dB 353.3 pA 300 mV 40.34 dB 5.149 nA 400 mV 40.89 dB 69.88 nA 600 mV 42.39 dB 4.478 µA 800 mV 42.86 dB 17.14 µA 810.6 mV 42.85 dB 17.44 µA 900 mV 42.73 dB 18.67 µA 1 V 42.55 dB 19.11 µA 1.2 V 41.89 dB 19.54 µA 1.274 V 41.26 dB 19.66 µA 1.4 V 39.43 dB 19.77 µA 1.6 V 38.7 dB 19.82 µA 1.8 V 38.64 dB 19.82 µA Below 0.811 V the tail current falls by decades, while the DC gain still looks fine. Check the tail current directly.
Self-Check
Tick each key point you covered in your answer.
What People Get Wrong
Common wrong answers and why they fail.
Misconception
"A follower has a gain of exactly one."
The source moves above the body, which raises the threshold and adds g_mb in the denominator. The lesson measured 0.841, not 1.
Source ASIC Point lesson E3.5 (measured)
Missed Corner
"A follower works for any input voltage."
The input must stay above V_GS plus the current sink's V_Dsat, or the sink leaves saturation and the output stops following.
Source ASIC Point lesson E3.5