How to subtract 2 signals without an opamp? (discrete or not)

Bricolo · 15th October 2005, 12:04 PM

Hi,

I'm looking for a way to subtract 2 signals, but using an opamp subtracter topology.
In fact, without feedback at all.

Does anyone have a clue?

Thanks

analog_sa · 15th October 2005, 12:11 PM

What is your application? Is a transformer an option?

Bricolo · 15th October 2005, 12:35 PM

I's for a subtractive crossiver project.
I'd prefer not to use transformers

johnnyx · 15th October 2005, 12:41 PM

Would a series crossover be ok?

MikeB · 15th October 2005, 12:44 PM

How big are the signals that need to be subtracted ? Linelevel or powersignals ?
Obviously, if you don't want to use any feedback, opamps are not welcome ?
A simple diffamp typically effectively subtracts two signals, that's why it is called diff-amp...

Or you use an "simple" inverter, and use additive resistors ?

Mike

Bricolo · 15th October 2005, 01:11 PM

It's for line level signals (IOW: active crossover, so no series one)

No feedback, you' right Mike.
a discrete open loop inverter (do you mean a common emitter with Rc=Re?) and an adder? Yes, that looks like a solution. I'll check that, thanks.

I remember having seen a "1 transistor" subtracter somewhere on this forum, but don't remember where exactly

. IIRC it has an input on the base, and the other at the collector (or emitter, don't remember exactly)

Christer · 15th October 2005, 01:17 PM

You can basically use a single BJT and use the base and emitter as inputs. However, using a diff pair will give a much much more linear solution. If you look at the transfer curve of a diff pair, you see that it is very linear within a certain voltage range and quickly becomes very non linear outside it (well, there is no sharp transition, of course). Emitter degeneration extends the linear region. You can either do the math, or just play with Spice and see what the transfer function looks like for various values of emitter degeneration.

Bricolo · 15th October 2005, 01:28 PM

in fact, charles alreadyd did everything I wanted to do

Active Subtractive XOs

15th October 2005, 12:04 PM	#1
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	How to subtract 2 signals without an opamp? (discrete or not) Hi, I'm looking for a way to subtract 2 signals, but using an opamp subtracter topology. In fact, without feedback at all. Does anyone have a clue? Thanks __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

15th October 2005, 12:35 PM	#3
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	I's for a subtractive crossiver project. I'd prefer not to use transformers __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

15th October 2005, 01:11 PM	#6
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	It's for line level signals (IOW: active crossover, so no series one) No feedback, you' right Mike. a discrete open loop inverter (do you mean a common emitter with Rc=Re?) and an adder? Yes, that looks like a solution. I'll check that, thanks. I remember having seen a "1 transistor" subtracter somewhere on this forum, but don't remember where exactly . IIRC it has an input on the base, and the other at the collector (or emitter, don't remember exactly) __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

15th October 2005, 01:28 PM	#8
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	in fact, charles alreadyd did everything I wanted to do Active Subtractive XOs __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

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15th October 2005, 12:11 PM	#2
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	What is your application? Is a transformer an option?

15th October 2005, 12:41 PM	#4
johnnyx diyAudio Member Join Date: Jan 2004 Location: manchester	Would a series crossover be ok?

15th October 2005, 12:44 PM	#5
MikeB diyAudio Member Join Date: Apr 2004 Location: Gütersloh	How big are the signals that need to be subtracted ? Linelevel or powersignals ? Obviously, if you don't want to use any feedback, opamps are not welcome ? A simple diffamp typically effectively subtracts two signals, that's why it is called diff-amp... Or you use an "simple" inverter, and use additive resistors ? Mike

15th October 2005, 01:17 PM	#7
Christer diyAudio Member Join Date: Sep 2002 Location: Sweden	You can basically use a single BJT and use the base and emitter as inputs. However, using a diff pair will give a much much more linear solution. If you look at the transfer curve of a diff pair, you see that it is very linear within a certain voltage range and quickly becomes very non linear outside it (well, there is no sharp transition, of course). Emitter degeneration extends the linear region. You can either do the math, or just play with Spice and see what the transfer function looks like for various values of emitter degeneration.

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