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Opamp vs single transistor buffer
Opamp vs single transistor buffer
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Old 15th February 2020, 10:32 AM   #1
Frederico Acardi is offline Frederico Acardi  Italy
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Default Opamp vs single transistor buffer

Hello. If you want a low impedance output to drive relatively moderate loads like 10K at 15Vpp AC.
EDIT : (The signal is 15Vpp max, but it is lower most of the time. )
(I precised 15Vpp 10K for the loading effect. It can be lower than 1Vpp.)

Is it "better" to use a single transistor source follower, or a op-amp buffer ?

I know that the single transistor will not provide unity gain, but can it be lower noise than an opamp ?
Not all opamp or transistor are the same and there is ones more noisy than other.

But as a general rule. Will the lowest noise transistor be lower noise than the lowest noise opamp ?

Thank you.

Last edited by Frederico Acardi; 15th February 2020 at 10:17 PM.
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Old 15th February 2020, 11:01 AM   #2
Mooly is offline Mooly  United Kingdom
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Opamp vs single transistor buffer
Interesting question and it seems that could have many many variables...

Have a read at this:
Emitter Followerl Noise Factor Calculator

Another characteristic of the single transistor is that you will not get DC precision, in other words it is more suited to AC coupled designs. The opamp on the other hand allows DC coupling and true zero volt output offset.
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Old 15th February 2020, 11:09 AM   #3
googlyone is offline googlyone  Australia
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Opamp vs single transistor buffer
"better" is a very subjective term.

As a unity gin buffer a fairly simple transistor circuit will do well. I am not sure that I would go right down to a single device (and even less certain that I would settle on nil feedback) but into 10K things are pretty simple.

Then again, an op amp buffer will do very well too. Noise in a unity gain application will be to all sane measures, "damn low".

What are you measuring this by? Parts count, Cost, Distortion, Noise, power consumption... The colour of the bits... all these lead to somewhat varying answers.

In my world, I would choose the op amp. If I had to use discrete, I would not be fussed, but it would not be "first choice"
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Old 15th February 2020, 03:28 PM   #4
Frederico Acardi is offline Frederico Acardi  Italy
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Thank you. If you had to go for lowest noise ?
I know that transistor are wide in tolerance, while an opamp is pretty much equal unit to unit. More predictable noise figures would be the opamp.

But if you only had to think in term of lowest noise and then lowest distortion, forget about cost, complexity, part count
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Old 15th February 2020, 04:09 PM   #5
MarsBravo is offline MarsBravo  Netherlands
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In my world however, I would go for a (2) discrete! A low noise jfet source follower with a jfet current source (same type & specs, matched) as load. Most simple adaption. Save the buffered opamp circuit alternative, of course.
Maybe a personal preference.
Y/Yo=(1-X/Xo)exp2 <> Y/Yo=(e)exp(X/Xo){Xo=kT/q}
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Old 15th February 2020, 04:33 PM   #6
analog_sa is offline analog_sa  Europe
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Opamp vs single transistor buffer
Cannot see noise being an issue at 15vpp with either reasonably low noise opamps or transistors. More like borderline impossible to measure.

As for a single transistor, this implies a coupling capacitor or transformer...

As already mentioned a two transistor circuit, either complementary or SE would work well, but dc stability if output is dc coupled, may be an issue.
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Old 15th February 2020, 05:12 PM   #7
JMFahey is offline JMFahey  Argentina
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At that point and signal level I seriously doubt noise is a problem.

Donīt overthink iot.
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
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Old 15th February 2020, 05:35 PM   #8
Frederico Acardi is offline Frederico Acardi  Italy
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Thank you.

But just to clarify everything is amplified after this buffer that is why I don't want exessive added noise.
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Old 15th February 2020, 08:34 PM   #9
adason is offline adason  United States
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Noise of the buffer will depend more on your power supply and shielding from hum then opamp or transistors used.
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Old 15th February 2020, 10:57 PM   #10
sandrohv is offline sandrohv  United States
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Hi Federico, this is a pretty bizarre question.

First, as Analog_SA and JMFahey pointed out, if the signal going through the follower is 15Vpp AC, why are you worrying about noise? A 10nV/rtHz follower circuit with 1MHz BW will result in 10uVRMS or ~14uVpp of noise. This is 120dB SNR, which is really good, more than what you can measure unless you have expensive equipment.

Now, let's say that maybe you made a mistake in stating the problem, the signals are much smaller and noise is an issue. If power is not a constraint, you can achieve the same noise with an op-amp as with the Emitter follower. The op-amp will consume a lot more current though to achieve the same noise level. BUT, the op-amp will offer many benefits like better DC precision, higher input Z, lower output Z, etc.

Other trade-offs are cost and ease of use. An op-amp will cost you more than an EF. Also if the op-amp is an SMT it is more difficult to solder (I myself can solder a SOIC, but anything smaller I cannot do). Finally, you may need extra power supplies if you don't have +/-15V or lower supplies.

I hope this helps.
Best, Sandro

Last edited by sandrohv; 15th February 2020 at 11:00 PM.
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