3 Step Gain Change, Op-amp with a transistor in the feedback loop

[Billy Mayo]

What is a 3 step gain change mean?

The op-amp as a transistor in the feedback loop

The op-amps output is at -100millivolts, the transistor is OFF

Once the signal is above -100 millivolts the transistor starts to conduct, but it does a 3 step gain change which mean?

 

[audioguru]

A 3 step gain change is when something switches resistors in the feedback loop to change the gain 3 gain amounts. A transistor can switch 2 gain steps: one step when the transistor is turned on and the second step when the transistor is turned off. Adding a second transistor will allow the gain to have 4 steps.

 

[Billy Mayo]

Thanks for the Help

There is 3 resistors around the transistor in the feedback loop

Step#1 is Resistor#1 + Transistor ON
Step#2 is Resistor + Transistor OFF?
Step#3 is Resistor#1 + Resistor#2 + Transistor ON?

The Transistor is some how switching the 3 resistors in the feedback loop? but how does it do it in a 3 step gain?

 

[audioguru]

One transistor has two states, on or off. Then the gain can have only 2 steps, not three steps.

 

[Billy Mayo]

But it says 3 steps in the service manual at work , one transistor and is has 3 resistors

Some how the transistor is doing 3 steps but how?

Transistor OFF is not a step right?

 

[ChrisP58]

Billy, post your circuit.

 

[audioguru]

Transistor OFF is not a step right?

Of course "transistor off" is a step. When the transistor is on then it shorts a resistor in the negative feedback to change the gain to the second step.
There is no 3rd step.

 

[Billy Mayo]

So how would you get the 3rd step?

 

[audioguru]

So how would you get the 3rd step?

Add a second transistor and another resistor.

 

[Billy Mayo]

A second transistor in parallel or in series?

 

[Billy Mayo]

Why would a circuit want to use a 3 step gain section? what are they mostly used for ? to do what?

 

[ChrisP58]

Why would a circuit want to use a 3 step gain section? what are they mostly used for ? to do what?

To allow it to work over different ranges.

For example. A multimeter has multiple ranges of voltage that it can measure, selected either by a manual rotary switch or automatic gain change. But your multimeter probably has more than three steps. Similarly, your o'scope also has a multi step input switch to give you different volts per division, as well as different sweep rates.

 

[Billy Mayo]

Most op-amps just have a feedback resistor to adjust the gain. Is this a One step gain? only at one set range?

What you mean it work over different ranges? you put 100mV input and the output is the different how so using a 3 step gain?

 

[audioguru]

If the feedback resistor for an opamp is two resistors in series then with both the gain is high.
When a turned on transistor shorts one resistor then the gain is reduced.

 

[Billy Mayo]

yes i can see that

When the transistor is OFF the gain is HIGH
When the transistor is ON the gain is Reduced

Any reason why a design would want to use this? a 2 step gain or 3 step gain section?

 

[ChrisP58]

yes i can see that

When the transistor is OFF the gain is HIGH
When the transistor is ON the gain is Reduced

Any reason why a design would want to use this? a 2 step gain or 3 step gain section?

So that whatever is controlling the transistor can change the gain. An auto ranging product will have a variable gain, and/or variable attenuation, in front of it's data converter.

 

[Billy Mayo]

Yes a variable gain is a Pot in the feedback of an op-amp to get a variable gain

But I'm not use to seeing a transistor in the feedback of an op-amp to do STEPS

It want's the control the transistors , but it wants 3 different FIXED GAINS

 

[ChrisP58]

Yes a variable gain is a Pot in the feedback of an op-amp to get a variable gain

But I'm not use to seeing a transistor in the feedback of an op-amp to do STEPS

It want's the control the transistors , but it wants 3 different FIXED GAINS

Billy, let me repeat what I said in post 6, post your circuit. Or at least tell us what it is part of. All of this guessing is just wasting time, because it doesn't tell you why your circuit is done the way it is.