# voltage follower explanation?

Discussion in 'General Electronics Chat' started by eng_alaakaa, Oct 8, 2011.

1. ### eng_alaakaa Thread Starter New Member

Sep 21, 2011
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what's the function of the
V3,R2 , R3, R1 and C1 ?

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2. ### Mike33 AAC Fanatic!

Feb 4, 2005
349
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Good question!
I think they're trying to show a split power supply with the extra battery and resistors (voltage divider)?...sort of an odd way of going about this, IMHO!
Maybe someone else has another take on it?

C1 acts as a frequency-dependent resistance...it passes frequencies above a certain threshold to ground. It is a filter. Anything above the cutoff frequency is trimmed off, although not sharply. < I THINK. not familiar with that opamp...

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3. ### Wendy Moderator

Mar 24, 2008
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Nope. A op amp is the basis for a linear power supply regulators, this is it taken to the simplest level. I use the configuration all the time for a virtual ground.

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4. ### eng_alaakaa Thread Starter New Member

Sep 21, 2011
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this is the 2nd stage of my receiver after envelop detector , but my question is why there's a voltage of 5V with two 1k ohm ?

5. ### Wendy Moderator

Mar 24, 2008
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I took another look at the design, it does seem superfluous with the extra power supplies. Way too many power supplies. Care to show the total schematic?

6. ### SgtWookie Expert

Jul 17, 2007
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R1/C1 comprise a Boucherot cell to suppress oscillations. [eta] Actually, not a Boucherot cell as Ron_H points out below; it's simply a low-pass filter.

The 5v source with two 1k resistors across it result in the input being biased @ 2.5v; with 2.5mA flowing through R2 and R3. Perhaps they thought it was easier to show it that way instead of using 3.8k for R2; as 3.8k is not a standard value of resistance.

Last edited: Oct 8, 2011
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7. ### eng_alaakaa Thread Starter New Member

Sep 21, 2011
12
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thank you all ^_^

8. ### Mike33 AAC Fanatic!

Feb 4, 2005
349
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I totally nose-dived on that one, ha ha. For the most part.
After I posted and went away, I thought about what I was actually looking at. SgtWookie actually knows what it's called!! You learn something every day on here, lol.

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9. ### Ron H AAC Fanatic!

Apr 14, 2005
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I think R1 and C1 are just a low pass filter, intended to keep high frequency noise from getting onto the 12V supply, and vice versa.
A Boucherot cell is a series RC network which is applied to the output of an amplifier to prevent high frequency oscillations which might occur if the amplifier were unloaded at high frequencies. Loudspeakers typically look inductive at high frequencies. The RC network in parallel with the speaker keeps the impedance and the phase shift of the load low at high frequencies.
Of course, the Boucherot cell has the same topology as R1C1, but the function is different. Also, the node between R and C in a Boucherot cell is typically not connected to anything else, as it is in this circuit.

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10. ### SgtWookie Expert

Jul 17, 2007
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Thanks for the correct spelling of Boucherot, Ron - I hadn't had any coffee before making that post, and I was too lazy to look it up.

The 0.1uF cap is required for bypassing of the opamp's supply. The 10 Ohm resistor helps reduce the noise that the opamp's supply might otherwise place on the 12v supply.

In an ideal circuit such as a PSPICE simulator, voltage sources have zero impedance (unless specified otherwise), unlike real-world voltage supplies that may have low impedance, but certainly not zero impedance. So, in a rudimentary PSPICE simulation, parts like C1 and R1 are often omitted and the impedance of the voltage supply left at zero to reduce the required run time of the simulation; the more complex the circuit, the longer it takes to compute all of the values.

11. ### Ron H AAC Fanatic!

Apr 14, 2005
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You're welcome, but the point I was making is that the circuit is not a Boucherot cell. The purpose is entirely different.

12. ### SgtWookie Expert

Jul 17, 2007
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Yes, I caught that, and agree - it's really a low-pass filter. I suppose I should go and correct my initial reply.