Question about differential amplifier configurations

[Space Varmint]

If I use a two transistor differential amplifier but use one core with bifilar windings for both transistors, will I have obtained "common mode" rejection?

 

[crutschow]

Have no idea what you are trying to do. How is the transformer connected to the transistors. Need a schematic.

 

[Willbe]

With a brain as big as in the photo, why aren't we all receiving your telepathically transmitted schematic?
Wrong freq?
Wrong modulation scheme?

 

[Space Varmint]

With a brain as big as in the photo, why aren't we all receiving your telepathically transmitted schematic?
Wrong freq?
Wrong modulation scheme?

LOL! OK, I get the message. I will input the schematic and forward it in broadband electro-tech PWM.

 

[Space Varmint]

Alright, here it is. This is one core.

 

[Nigel Goodwin]

As there's only one input, how can you expect to get common mode rejection anyway?.

 

[Space Varmint]

As there's only one input, how can you expect to get common mode rejection anyway?.

That's a diff amp. That's what they do. Only I don't believe this is a true diff amp because of the sharing of the inductor. See I did notice a phase reversal between using two coils. So I'm thinking I may have lost my noise reduction (common mode).

 

[crutschow]

Common mode noise is the common-mode (in-phase) noise on two signal carrying wires (typically carrying a differential-mode (out-of-phase) signal). It has no meaning for a single input.

A diff amp can suppress this noise, but only if both inputs are used for the differential signal. A diff amp has no signal noise suppression for a single-ended signal.

A diff amp can suppress noise from the power supply if it's part of a differential signal channel, but that's not usually considered common-mode noise.

 

[Nigel Goodwin]

That's a diff amp. That's what they do. Only I don't believe this is a true diff amp because of the sharing of the inductor. See I did notice a phase reversal between using two coils. So I'm thinking I may have lost my noise reduction (common mode).

Like I said above, you can't have common-mode rejection on a single input, it requires two complementary inputs.

 

[Space Varmint]

Common mode noise is the common-mode (in-phase) noise on two signal carrying wires (typically carrying a differential-mode (out-of-phase) signal). It has no meaning for a single input.

A diff amp can suppress this noise, but only if both inputs are used for the differential signal. A diff amp has no signal noise suppression for a single-ended signal.

A diff amp can suppress noise from the power supply if it's part of a differential signal channel, but that's not usually considered common-mode noise.

Then why are there single input diff amps? The one I drew in the schematic, if it had two coils then I know it is a diff amp.

 

[Roff]

Then why are there single input diff amps? The one I drew in the schematic, if it had two coils then I know it is a diff amp.

A diff amp by definition amplifies the difference between two inputs. You are probably thinking of an amplifier with non-inverting and inverting outputs. While it's true that this is sometimes called a differential output amplifier, the term differential amplifier is generally refers to one having differential inputs.

 

[Space Varmint]

A diff amp by definition amplifies the difference between two inputs. You are probably thinking of an amplifier with non-inverting and inverting outputs. While it's true that this is sometimes called a differential output amplifier, the term differential amplifier is generally refers to one having differential inputs.

OK, that must be it. Appreciate it and you too Nigel. That's why I was here asking the question. Answer: No noise reduction! Oh well...lol.

 

[Roff]

OK, that must be it. Appreciate it and you too Nigel. That's why I was here asking the question. Answer: No noise reduction! Oh well...lol.

Noise from where/what?

 

[MrAl]

Hi,

There is probably no reason why you can't change the circuit around a little
to get it to work as a diff amplifier and obtain some common mode noise
canceling...im sure someone here would be happy to draw up a schematic
starting from the one you already have.

 

[Space Varmint]

Hi,

There is probably no reason why you can't change the circuit around a little
to get it to work as a diff amplifier and obtain some common mode noise
canceling...im sure someone here would be happy to draw up a schematic
starting from the one you already have.

Hey, appreciate that. I was asked about the noise. What I am doing is using this configuration as the first amp in an AGC loop. So, yeah, any noise I can shake is most desirable. At least, and correct me if I'm wrong, but I will get twice the power out of the stage and I found that small changes on the base will cause large swings in the amplitude of the signal output.

 

[Nigel Goodwin]

Hey, appreciate that. I was asked about the noise. What I am doing is using this configuration as the first amp in an AGC loop. So, yeah, any noise I can shake is most desirable. At least, and correct me if I'm wrong, but I will get twice the power out of the stage and I found that small changes on the base will cause large swings in the amplitude of the signal output.

You're still missing the point, common-mode rejection requires two complementary inputs, it then rejects anything common to both inputs, leaving just the signal you wanted. It's used to reject interference picked up directly on the leads - like on a balanced microphone - it's a balanced system.

It has no bearing on what you're trying to do (whatever it is).

 

[ecerfoglio]

You're still missing the point, common-mode rejection requires two complementary inputs, it then rejects anything common to both inputs, leaving just the signal you wanted. It's used to reject interference picked up directly on the leads - like on a balanced microphone - it's a balanced system.

It has no bearing on what you're trying to do (whatever it is).

Perhaps he is thinking about "Power supply noise rejection": the hability of the amplifier to remain "quiet" even if the supply is noisy....

 

[Space Varmint]

You're still missing the point, common-mode rejection requires two complementary inputs, it then rejects anything common to both inputs, leaving just the signal you wanted. It's used to reject interference picked up directly on the leads - like on a balanced microphone - it's a balanced system.

It has no bearing on what you're trying to do (whatever it is).

No, I realize that but I had some strange idea that the single ended configuration I was using would help kill noise. I think I read it wrong...lol. Maybe I can lie my way out of this one.

 

[Space Varmint]

But explain to me this then. Why do they use these on the input of allot of chips? What do they do to get the extra power gain? They don't feed both base's do they?

 

[Nigel Goodwin]

But explain to me this then. Why do they use these on the input of allot of chips? What do they do to get the extra power gain? They don't feed both base's do they?

It's used to create a differential signal from a single ended one, or to give higher gain - two transistors give more gain than one.