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Questions tagged [source-follower]

A source follower (also known as a common drain amplifier) is one of three basic single-stage field effect transistor (FET) amplifier topologies, typically used as a voltage buffer. Use this tag along with the tag for the type of FET (e.g. "mosfet").

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Unity Gain Amplifier Purpose

In layman terms, what is the main use and purpose of unity gain amplifiers? I am reading that it is used to "buffer high impedance sources" but I'm unsure what this means practically.
RL1's user avatar
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2 answers
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How does a "regulating" source follower work?

Can someone explain to me how this circuit works? The output voltage is ~2.4V. The zener diode breakdown is 4.7V. From what I can see the OUT is simply Vgate - Vgs = 4.33V - 1.85V = 2.48V. Is this ...
JoeyB's user avatar
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2 votes
1 answer
170 views

Why will the offset of a source-follower which is connected to the output of an opamp as a voltage buffer limit the op-amp output swing?

I am currently reading chapter 12, "The Common-mode Feedback Circuit" in Analysis and Design of Analog Integrated Circuits by Gray and Hurst. Page 815, gives the following statement for ...
Tong Su's user avatar
  • 113
1 vote
0 answers
120 views

JFET + Bipolar source follower: with VGS constant, how Vout can be an AC signal?

Id is the sum of IB and IR1 with IB small. We can approximatively say Id equals IR1. BJT give an voltage across R1 equals to VBE. VR1 = VBE which is limiting the current flowing across R1. In ...
Acapulco's user avatar
0 votes
2 answers
221 views

How can I improve this source follower circuit?

Previously I asked for help with a power supply for titanium anodizing and my requirements were too vague. After doing some testing I believe I have found a schematic which I was able to use and ...
Infernoman's user avatar
0 votes
2 answers
805 views

What does source follower mode mean?

I've been trying to build a CMOS-level XOR gate. I've been told my implementation won't work because some of the transistors are in "source follower mode". I've tried looking this up, but I ...
Connor's user avatar
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1 answer
210 views

Source follower input/output swing

I found in this book at the page 271 a definition of the source follower input/output swing ( that i didn`t know as: $$ V_{in-pp} = V_{dd} - ( Vt + 2.V_{ds-sat} )$$ I tried to understand how to ...
Mxm89's user avatar
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0 answers
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What happens if the gate voltage exceeds the drain voltage in a source follower with an active load?

What happens if the gate voltage exceeds the drain voltage in a source follower with an active load? Here is the circuit: If we try to increase input voltage even more than Vdd, what happens then? ...
mohammad rezza's user avatar
-3 votes
3 answers
136 views

Puzzling behavior for op-amp driven MOSFET

Consider this circuit: here (Ctrl+S on the text view) As you can see, the positive input, \$V_{IN1}\$, of the op-amp is floating & yet, it will exhibit the following behavior: Just like a ...
Tempus Nomen's user avatar
0 votes
2 answers
64 views

Separating power ground from control ground to exceed NMOS threshold voltage

I'm a novice circuit designer looking to create a high voltage source follower NMOS circuit. I originally planned to use a Teensy digital I/O pin as the gate control signal, so when the Teensy ...
asyndeton256's user avatar
0 votes
2 answers
423 views

MOSFET analog output buffer for 0 V to 2.4 V signal

I have a signal that starts at 0 V and linearly increases to 2.4 V as output and I would like to buffer it such that it cannot be influenced by resistive loads. The issue is that the only buffer ...
Saens's user avatar
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2 votes
0 answers
2k views

Source-Follower Large Signal Analysis as Level Shifter

I have a question about the MOS source-follower / common-drain stage. In particular, it's behaviour as a level shifter: The book mentions the following: I'm having some trouble figuring out how the ...
AlfroJang80's user avatar
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2 votes
0 answers
252 views

Need help for pointing out, source follower circuit with current source, designing NMOS

I'm not sure did I solve this problem right. I made \$ r_{o2} \$ , which is resistance looking at Q2 drain. So I can get $$ g_{m,Q3} = 1.8m(A/V) $$ from $$A_v = \frac{r_{o2} || r_{o3}}{1/g_m + (r_{o2} ...
Pare Kanes's user avatar
1 vote
2 answers
2k views

Source Resistor of JFET Source-Follower

I am designing a simple Buffer to get the voltage of a high-impedanze source to an ADC (60k Impedance). For that I choosed an JFET in the source-follower configuration. simulate this circuit – ...
Feuerlink's user avatar
0 votes
1 answer
72 views

I would like to know if the circuit is correct?

simulate this circuit – Schematic created using CircuitLab I have used the voltage follower concept in order to keep the output logic non-inverted.I would like to know the issues that I might ...
Abhi's user avatar
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13 votes
4 answers
1k views

How does adding a current source and extra resistor to source follower improve its performance?

This question is similar to FET - Source follower utilizing current source to lower voltage offset, although I do not entirely understand the explanations, which is why I am posting my own question. ...
corgiworld's user avatar
3 votes
0 answers
194 views

Reducing the offset voltage of a source follower

I am currently trying to reduce the offset voltage of the circuit attached below to less than +/- 2mV over a 1V to 4V input range. I have a few constraints for this task: I can use ...
Oreoluwa Adesina's user avatar
1 vote
1 answer
158 views

Tuning a potential sensor using a source-follower MOSFET

I am interested in building a simple but sensitive setup that would allow me to detect small potential changes (adsorption of ions) on the surface of the gate of a MOSFET - what is commonly referred ...
Mowgli's user avatar
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0 answers
225 views

How can I reduce loading effect in dc common drain source follower?

I want to make a dc voltage buffer using an nmos source follower. I found that by making Rs as large as possible or even open, Vs= Vgs-Vth. I verified it using multisim. The problem is it suffers from ...
ElecNoob's user avatar
0 votes
1 answer
88 views

Can I use a source follower to read the floating gate voltage then add a fixed tunneling voltage, then feed it back to the tunneling electrode?

So I was reading this research paper: H. Kosaka, T. Shibata, H. Ishii, and T. Ohmi, “An excellent weightupdating-linearity synapse memory cell for self-leaming neuron MOS neural networks,” in IEDM ...
ElecNoob's user avatar
0 votes
1 answer
787 views

CMOS source follower with 2 in series

I am really struggling with where to start to analyse this. Using the Shichman Hodges Model and the small signal equivalent circuit, the voltage gain can be equation can be seen to be vo = -gmvgsr0. ...
John's user avatar
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0 votes
2 answers
3k views

What are the advantages and uses of a voltage follower circuit over emitter follower?

Can I use a voltage follower and emitter follower for same purpose?
mohammed rashid thattayil's user avatar
2 votes
1 answer
1k views

Source follower basic question

I just want to clear my understanding about a basic source follower circuit. Following is the circuit Here is my question: Considering Vi > Vgs(th); Vo = Vi - Vgs. Also Vo = Id*Rs as shown in the ...
RAN's user avatar
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2 answers
945 views

Stability problem OpAmp and FET source follower

I just found this site and read a few great interesting articles but nothing that would solve my problem I am hunting since a while. I have a OpAmp circuit with a N-FET source follower that regulates ...
6o4o's user avatar
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2 votes
3 answers
2k views

Strange output with a LM7171 opamp (non-inverser amplifier)

I have a continuous input signal with a very high impedance and I need an output signal with a low impedance of 50Ω. That's why I've considered the LM7171 (High Output Current: 100 mA). I've tried to ...
Luc's user avatar
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0 votes
1 answer
494 views

Follower current source?

I need advice on a problem I'm having I need a follower to transmit an AC signal into a low impedance load which is the cathode of a triode but I also need it to be an adjustable current source to ...
coinmaster's user avatar
2 votes
1 answer
1k views

Source Follower Output Biasing

I am trying to understand the Source Follower amplifier as a primitive buffer, and am running some simulations with a SPICE simulator. I have the amplifier hooked up as shown below with a current ...
Dr. Watson's user avatar