Transistors switch on output pin or input pin

[Davewalker5]

What does these transistors do? U2 , 2N2920

They are placed on the op amps output pin and another transistor is placed on the input of an op amp

It seems that these transistors are used as switches to clean up or switch off so the output pin of a op amps is not leaky noise or voltage when it's off. The transistors "sinks" the op amps output to ground so the next stage doesn't amplify or create a false trigger

Take a look at the schematic

 

[Arouse1973]

Hello
The first part is a log amp used for smoothing out very large variations in input signal to a more realistic level. Used in measuring light that varies from dull to very bright is one application. The second transistor is diode connected, its a diode basically and makes sure the input to the folowing stage is a diode drop above the input, this might be due to limitations in the opamp. Also using a transistor has better temperature tracking than a diode.

But not knowing the design this is only subjective.

Adam

 

[Davewalker5]

The first part is a log amp used for smoothing out very large variations in input signal to a more realistic level.

I thought a to make a log amp, was to put a transistor in the feedback loop not on the output pin

The second transistor is diode connected, its a diode basically and makes sure the input to the folowing stage is a diode drop above the input, this might be due to limitations in the opamp

So the Transistor is acting like a diode to "clamp" the input voltage? to limit it to not overload the input?

 

[Arouse1973]

It is across the output and input,look again.
Adam

 

[Davewalker5]

It is across the output and input,look again.

Yes true, but they are using the transistor like a diode? or a variable diode

 

[Arouse1973]

I have never seen a variable diode what do you mean.

 

[Davewalker5]

They are using the transistor like a variable diode right?

Transistor#1
Because the transistors base is grounded , they are just using the emitter and collector

Transistor#2
They tried the the base and collector to VCC
So they are using just the emitter to base internal diode inside the transistor

 

[Arouse1973]

The voltage across a diode is a log function of the current through it. The second diode keeps the input of the second opamp approx 0.6 Volts above the output of the first opamp. You will have to ask the desinger of the circuit as to why. But I guess it because of the opamps inputs

 

[Davewalker5]

The second diode keeps the input of the second opamp approx 0.6 Volts above the output of the first opamp.

Above or below? i was thinking it was below the output of the first op amp not above

The voltage across a diode is a log function of the current through it.

Yes true

You will have to ask the desinger of the circuit as to why. But I guess it because of the opamps inputs

explain more about your theory, the op amps inputs are? or don't have?

I was thinking the transistor is just "clamping" the op amps input

 

[Arouse1973]

Some op-amps can not take voltages too close to either supply rail, so I am guessing this is ensuring that the input of the op-amp always sits 0.6 Volts above the input signal. But I didn't design it and as we can only see part of the circuit and we don't know what it does.
Thanks
Adam

 

[KrisBlueNZ]

It's for temperature compensation. The V_BEs of the two transistors are supposed to track each other fairly closely. The transistors may be strapped together.

Edit: Yes it would be helpful to see the whole circuit and to know what it does.

You (Davewalker5) have posted at least five snippets of this circuit. We could help you a lot more if we saw the whole circuit and knew what it was from.

 

[Arouse1973]

Yes Kris this did cross my mind about temperature tracking but how poorly implemented it is I discounted it. The compensation done this way I think should be from a constant current source or in the feedback loop of the log amp. Another option I have also seen is a PNP transdiode on the output. But not seeing the rest of the circuit I wasn't sure if it has this compensation elsewhere.
Cheers
Adam

 

[Davewalker5]

It's for temperature compensation.

How are these transistors temperature compensation? how so ? and when

When the current gets to hot and warm for being left on for hours ? or when current gets higher in the current?

about temperature tracking

Tracking what? how are transistors tracking

 

[KrisBlueNZ]

As I said in post #11:

The V_BEs of the two transistors are supposed to track each other fairly closely. The transistors may be strapped together.

Many transistor parameters vary with temperature. V_BE is just one of them, but it's the one being compensated here.

If both transistors are the same type, and they're both at the same temperature, their V_BE voltages should also be the same.

What is this circuit from?
Can you post the whole schematic?

 

[Davewalker5]

What is this circuit from?
Can you post the whole schematic?

I'm trying to find the schematic, mostly i just got snippets on my camera of parts of schematics which i thought i didn't know how it worked

I still not sure what u mean by this tracking? what do u mean by tracking

Many transistor parameters vary with temperature. VBE is just one of them, but it's the one being compensated here.

Yes i know , because germanium transistors are worse with temperature varying in the VBE junction

If both transistors are the same type, and they're both at the same temperature, their VBEvoltages should also be the same.

Each transistors VBE is not a clone

I would think they would be offset in voltage, not in union together

 

[KrisBlueNZ]

I'm trying to find the schematic, mostly i just got snippets on my camera of parts of schematics which i thought i didn't know how it worked

I don't believe you.

I still not sure what u mean by this tracking? what do u mean by tracking

The V_BE voltages are both dependent on the same quantities - transistor type and temperature. So as long as they are the same transistor type (from the same batch) and are kept at the same temperature, their V_BE voltages may vary significantly, but they will always be roughly equal. They track each other.

Each transistors VBE is not a clone

Two transistors from the same batch at the same temperature will track pretty well.

I would think they would be offset in voltage, not in union together

Well thanks for sharing.

 

[Davewalker5]

Here is a snip of a schematic which actually has transistors used as switches on the output pin of a Op amp comparator

Any reason why they would use these transistors on the output pin of an op amp comparator?

To make sure there is no leakage on the output pin? the transistors switch sinks the output pin of the comparator, but it would have sinked any ways without the transistor right?

take a look please

 

[KrisBlueNZ]

My guess would be that they only wanted the circuit to sink current and not source current. They could have used a diode but they may have needed more current than the 709 could provide, so they used emitter followers instead of a diode.

Without a lot more context it's impossible to say.

What is the circuit from?
Can you post the whole thing?

 

[Davewalker5]

My guess would be that they only wanted the circuit to sink current and not source current.

If there was no transistor on the output pin

The output voltage and current would source? or sink?

Or how would it source?

They could have used a diode but they may have needed more current than the 709 could provide, so they used emitter followers instead of a diode.

Yes true , i have seen this

 

[(*steve*)]

The two transistors are part of the same package. The second transistor might be able to be replaced with a diode, but it is at the same temperature as the first transistor. This means that the Vbe drops are identical.