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[SOLVED] Maximum feedback resistor value for biasing an amplifier input

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CHL

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Hello

I have a question about a feedback resistor value.

To bias an input of an amplifier, we can use a feedback resistor from output to input.

Normally we use a high resistance, but there may be problems.

1) Amplifier input impedance
If the feedback impedance is higher than the input impedance of the amplifier, input currents may flow through the amplifier input pin.

2) Open circuit
If the resistance value is too high, it is like an open circuit. so although there is a feedback, it may be an open circuit.

I didn't mention a stability issue since it can be compensated by using a capacitor.

So, how to decide the maximum feedback resistance value?

Thanks.
 

It's a tradeoff between a number of conflicting requirements.
You select the value that is a best compromise between all the requirements.
That's where the engineering comes in.
 

If you use a very high negative feedback resistance then the stray capacitance at the (-) input causes a phase shift that might cause oscillation.
If you need an opamp with a very high input impedance then use one with Fet inputs. Many are available.
 

Can you tell me more specifically?

I understand there is a trade-off, but I want to know more details

Thanks

- - - Updated - - -

I forgot to think about a stray cap. Thanks

However, when we design an inverting amplifier or a charge amplifier, we need a feedback resistor.

So, I want to know what kind of things I need to consider when I decide the resistance value.
 

However, when we design an inverting amplifier or a charge amplifier, we need a feedback resistor.
Click to expand...

In that case, if both your input impedance and feedback resistor may be extremely high impedances, then unintended leakage resistance to other parts of the circuit may be your worst problem.

In that case, you may need to place a guard ring around your inverting input, and connect it to a voltage that tracks the inverting input.
usually that will be the non inverting input, which hopefully is tied down to a much lower impedance.

Another way is to bend the leg of the IC outwards, and solder straight onto that, and not solder that pin to a pad on the board. Its a bit ugly, but very practical if trying to avoid stray leakage currents in a harsh environment.
 
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CHL said:
Normally we use a high resistance, but there may be problems.
Click to expand...

I suppose, you are speaking about a BJT common-emitter stage?
In thius case, you have no real choice.
The voltage at the collector node (output) is - more or less - fixed.
And the necessary base voltage is 0.65...0.7 volts.
Hence, the feedback resistor is determined by the desired base current.
The situation changes in case of a voltage divider at the base. Now you can select a certain impedance niveau (trade-off).
 
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However, when we design an inverting amplifier or a charge amplifier, we need a feedback resistor.

So, I want to know what kind of things I need to consider when I decide the resistance value.
Click to expand...
You are asking about two different amplifier topologies in one question, charge amplifier and inverting voltage amplifier, they have different requirements for the feedback resistor. Asking too general brings up misunderstandings and less helpful answers.

For a reasonable discussion, we should specify some conditions, at least
- signal source impedance
- amplifier input impedance and bias current
- amplifier noise parameters
- amplifier gain and frequency characteristic

You can sketch equivalent circuits of both amplifier topologies and calculate the effect of feedback resistor variation.
 
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... and don't forget there are also 'current feedback' amplifiers that are designed to sink significant current into their input pins in normal operation. They are normally used in low impedance circuits (video amps for example) and the feedback resistor often can't be higher than a few K Ohms.

Brian.
 
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Hi,

Hi,

I'm a bit confused.
With the feedback resistor (from output to the inverting input) you set the gain of the amplifier.

How can the feedback resistor be used for input biasing?

Klaus
 

Hi,

Klaus - what about a common emitter amplifier with a resistor between collector and base?
Click to expand...

Oh yes, thanks, I had an Opamp in mind...

Klaus
 

There's not principle difference between single transistor amplifier and OP in inverting configuration in this regard. Both need the feedback resistor also to set the DC operation point.
 
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From an analog perspective it's always good (for a regular
voltage mode op amp) to make the pin impedances match
for minimum Vio. Otherwise you have an Iib*(Rpos-Rneg)
adder term to input offset voltage. This will tend to make
your feedback resistor value not the primary concern other
than a unity gain follower, and even there you'll match
another quantity than "what's the biggest" - like the input
sensor's natural source impedance, or as-modified by any
input network.
 
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KlausST said:
With the feedback resistor (from output to the inverting input) you set the gain of the amplifier.

How can the feedback resistor be used for input biasing?
Click to expand...

You can always separate ac and DC gain by a capacitor.
 

An opamp has two inputs. It has the non-inverting input (+) that needs a bias reference voltage (sometimes not a current) and it has the inverting input (-) that has the negative feedback resistor.
 

Hi,

An opamp has two inputs. It has the non-inverting input (+) that needs a bias reference voltage (sometimes not a current) and it has the inverting input (-) that has the negative feedback resistor.
Click to expand...
That's the way I see it.
Operation point is set by the noninverting input...but the feedback resistor goes to the inverting input.

Klaus
 

KlausST said:
Hi,
That's the way I see it.
Operation point is set by the noninverting input...but the feedback resistor goes to the inverting input.
Klaus
Click to expand...
I think, the DC operational point (DC output voltage) is Dc offset voltage multiplied by noise gain (1+Rf/Ro).
Hence, DC feedback reduces the DC offset at the output and, thus, defines the operational point.
 
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First you need to see all other parameters- e.g., bandwidth and frequency response. If you set your gain at arbitrary value, whether these are going to be affected. Then you decide whether you need to split the gain in several stages. These points have been raised in #7 post and I agree with all of them except that they are in reverse (I think so). First you consider your required bandwidth and frequency response and then you consider the noise parameters. These parameters are application dependent. You can certainly get a high gain at the cost of other parameters of significance (if you are planning to go in a single stage).

My suggestion is you divide the problem in several stages and your problem is poorly specified. For every amplifier gain, bandwidth and noise are related. There are operational amplifiers with FET and MOSFET inputs that offer very low input current. However, that does not imply that you can have arbitrarily high gain. Just consider a common radio receiver that amplifies an input signal (of the order of 1uV) by approx million times. In an NMR instrument, the gain is perhaps 100-1000 times more but these are done in stages.
 
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If the resistance is very high, little current can flow. In this case, the circuit becomes like an open circuit. How can I find the minimum required feedback current through the feedback resistor?(for example, a charge amplifier case)
pA range or fA range? or should it be the input bias current of the opamp?
 

The input bias current in the feedback resistor (and maybe also in its other resistor) causes a voltage shift in the output voltage if the opamp. Usually an opamp has an extremely low input bias current so the effect is very small.
If your circuit needs a 10M feedback resistor and you do not mind having a voltage shift of 0.1V (when the other resistor is capacitor-coupled to ground) then a bias current of 0.1V/10M= 10nA will do it.
 
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