Forums

Replacing a transformer with opamps

Started by George Herold November 10, 2011
I=92ve been mucking about with double balanced mixers.
I wanted something to work below 100kHz, so I ordered
a few audio transformers.  They worked fine, but it seemed
to me that you should be able to replace the transformers
with some opamps.  I bashed my head against the wall* for a
day or so trying to come up with a circuit of my own.
Today I went searching online and found this idea from Tom Hay.
Basically two cross-connected differential amps with the noninverting
input =91ground=92 coming from the output of the other opamp.
Slick!  (LT spice file attached)

Any other ideas on how to do this?
Maybe there is already an IC that does this?

George H.

*Sometimes I find something, but most of the time I just put a dent in
the wall.

Version 4
SHEET 1 880 1008
WIRE -544 -176 -576 -176
WIRE -384 -176 -464 -176
WIRE -352 -176 -384 -176
WIRE -240 -176 -272 -176
WIRE -576 -144 -576 -176
WIRE -384 -80 -384 -176
WIRE -336 -80 -384 -80
WIRE -240 -64 -240 -176
WIRE -240 -64 -272 -64
WIRE -208 -64 -240 -64
WIRE -96 -64 -128 -64
WIRE 320 -64 -96 -64
WIRE 416 -64 320 -64
WIRE 528 -64 464 -64
WIRE -560 -48 -704 -48
WIRE -496 -48 -560 -48
WIRE -384 -48 -416 -48
WIRE -336 -48 -384 -48
WIRE 320 -32 320 -64
WIRE 416 -32 416 -64
WIRE -384 32 -384 -48
WIRE -352 32 -384 32
WIRE -128 32 -272 32
WIRE -704 48 -704 -48
WIRE 320 48 320 32
WIRE 320 48 256 48
WIRE 416 48 416 32
WIRE 464 48 464 -64
WIRE 464 48 416 48
WIRE 528 48 528 -64
WIRE -480 128 -512 128
WIRE -368 128 -400 128
WIRE -352 128 -368 128
WIRE -96 128 -96 -64
WIRE -96 128 -272 128
WIRE -704 144 -704 128
WIRE -704 144 -816 144
WIRE -512 160 -512 128
WIRE 528 160 528 128
WIRE 768 160 528 160
WIRE -816 176 -816 144
WIRE -128 176 -128 32
WIRE -16 176 -128 176
WIRE 64 176 64 96
WIRE 64 176 -16 176
WIRE 192 176 64 176
WIRE 192 192 192 176
WIRE 768 192 768 160
WIRE -704 208 -704 144
WIRE -368 208 -368 128
WIRE -336 208 -368 208
WIRE 528 208 528 160
WIRE -240 224 -272 224
WIRE -208 224 -240 224
WIRE -128 224 -128 176
WIRE -560 240 -560 -48
WIRE -480 240 -560 240
WIRE -368 240 -400 240
WIRE -336 240 -368 240
WIRE 768 288 768 272
WIRE 192 304 192 272
WIRE -368 320 -368 240
WIRE -336 320 -368 320
WIRE -240 320 -240 224
WIRE -240 320 -256 320
WIRE 320 320 320 48
WIRE 416 320 416 48
WIRE 320 400 320 384
WIRE 320 400 64 400
WIRE 416 400 416 384
WIRE 416 400 320 400
WIRE 256 432 256 48
WIRE 528 432 528 288
WIRE 528 432 256 432
WIRE -544 464 -576 464
WIRE -384 464 -464 464
WIRE -352 464 -384 464
WIRE -240 464 -272 464
WIRE -576 496 -576 464
WIRE -384 560 -384 464
WIRE -336 560 -384 560
WIRE -240 576 -240 464
WIRE -240 576 -272 576
WIRE -208 576 -240 576
WIRE -96 576 -128 576
WIRE 64 576 64 400
WIRE 64 576 -96 576
WIRE -496 592 -560 592
WIRE -384 592 -416 592
WIRE -336 592 -384 592
WIRE -384 672 -384 592
WIRE -352 672 -384 672
WIRE -128 672 -272 672
WIRE -704 704 -704 288
WIRE -560 704 -560 592
WIRE -560 704 -704 704
WIRE -480 768 -512 768
WIRE -368 768 -400 768
WIRE -352 768 -368 768
WIRE -96 768 -96 576
WIRE -96 768 -272 768
WIRE -512 800 -512 768
WIRE -128 832 -128 672
WIRE -16 832 -16 176
WIRE -16 832 -128 832
WIRE -368 848 -368 768
WIRE -336 848 -368 848
WIRE -240 864 -272 864
WIRE -208 864 -240 864
WIRE -128 864 -128 832
WIRE -560 880 -560 704
WIRE -480 880 -560 880
WIRE -368 880 -400 880
WIRE -336 880 -368 880
WIRE -368 960 -368 880
WIRE -336 960 -368 960
WIRE -240 960 -240 864
WIRE -240 960 -256 960
FLAG 192 304 0
FLAG 768 288 0
FLAG -816 176 0
FLAG -576 -144 0
FLAG -512 160 0
FLAG -576 496 0
FLAG -512 800 0
FLAG 64 96 Vout
SYMBOL voltage -704 32 R0
WINDOW 123 0 0 Left 2
WINDOW 39 -157 61 Left 2
WINDOW 3 -195 24 Left 2
SYMATTR SpiceLine Rser=3D10
SYMATTR Value SINE(0 1 100k)
SYMATTR InstName V1
SYMBOL diode 400 -32 R0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL diode 400 320 R0
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL diode 336 384 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D3
SYMATTR Value 1N4148
SYMBOL diode 336 32 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D4
SYMATTR Value 1N4148
SYMBOL voltage 528 32 R0
WINDOW 123 0 0 Left 2
WINDOW 39 92 72 Left 2
WINDOW 0 79 13 Left 2
WINDOW 3 77 47 Left 2
SYMATTR SpiceLine Rser=3D10
SYMATTR InstName V2
SYMATTR Value SINE(0 .5 10k)
SYMBOL res 208 288 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R3
SYMATTR Value 1k
SYMBOL res 784 288 R180
WINDOW 0 -39 78 Left 2
WINDOW 3 -47 33 Left 2
SYMATTR InstName R4
SYMATTR Value 1
SYMBOL voltage -704 192 R0
WINDOW 123 0 0 Left 2
WINDOW 39 -222 31 Left 2
WINDOW 0 -178 103 Left 2
WINDOW 3 -242 65 Left 2
SYMATTR SpiceLine Rser=3D10
SYMATTR InstName V3
SYMATTR Value SINE(0 1 100k)
SYMBOL voltage 528 192 R0
WINDOW 123 0 0 Left 2
WINDOW 39 54 95 Left 2
WINDOW 0 75 4 Left 2
WINDOW 3 52 49 Left 2
SYMATTR SpiceLine Rser=3D10
SYMATTR InstName V4
SYMATTR Value SINE(0 .5 10k)
SYMBOL Opamps\\opamp -304 -128 R0
SYMATTR InstName U1
SYMBOL Opamps\\opamp -304 288 M180
SYMATTR InstName U2
SYMBOL res -512 -32 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R5
SYMATTR Value 10k
SYMBOL res -368 -160 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R6
SYMATTR Value 10k
SYMBOL res -352 336 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R7
SYMATTR Value 10k
SYMBOL res -496 256 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R8
SYMATTR Value 10k
SYMBOL res -560 -160 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R9
SYMATTR Value 10k
SYMBOL res -496 144 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R10
SYMATTR Value 10k
SYMBOL res -368 144 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R11
SYMATTR Value 10k
SYMBOL res -368 48 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R12
SYMATTR Value 10k
SYMBOL res -224 -48 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R13
SYMATTR Value 10
SYMBOL res -224 240 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R14
SYMATTR Value 10
SYMBOL Opamps\\opamp -304 512 R0
SYMATTR InstName U3
SYMBOL Opamps\\opamp -304 928 M180
SYMATTR InstName U4
SYMBOL res -512 608 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R1
SYMATTR Value 10k
SYMBOL res -368 480 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL res -352 976 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R15
SYMATTR Value 10k
SYMBOL res -496 896 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R16
SYMATTR Value 10k
SYMBOL res -560 480 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R17
SYMATTR Value 10k
SYMBOL res -496 784 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R18
SYMATTR Value 10k
SYMBOL res -368 784 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R19
SYMATTR Value 10k
SYMBOL res -368 688 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R20
SYMATTR Value 10k
SYMBOL res -224 592 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R21
SYMATTR Value 10
SYMBOL res -224 880 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R22
SYMATTR Value 10
TEXT 456 568 Left 2 !.tran 400u
TEXT 464 624 Left 2 !.include opamp.sub
On Thu, 10 Nov 2011 09:04:45 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>I&#2013266066;ve been mucking about with double balanced mixers. >I wanted something to work below 100kHz, so I ordered >a few audio transformers. They worked fine, but it seemed >to me that you should be able to replace the transformers >with some opamps. I bashed my head against the wall* for a >day or so trying to come up with a circuit of my own. >Today I went searching online and found this idea from Tom Hay. >Basically two cross-connected differential amps with the noninverting >input &#2013266065;ground&#2013266066; coming from the output of the other opamp. >Slick! (LT spice file attached) > >Any other ideas on how to do this? >Maybe there is already an IC that does this? > >George H.
There's the simple +1/-1 opamp thing, discussed here before. Just a few parts. The only gotcha is that the LO is essentially a square wave. John
"George Herold" <gherold@teachspin.com> wrote

<QUOTE>
I&#2013266066;ve been mucking about with double balanced mixers.
I wanted something to work below 100kHz, so I ordered
a few audio transformers.  They worked fine, but it seemed
to me that you should be able to replace the transformers
with some opamps.  I bashed my head against the wall* for a
day or so trying to come up with a circuit of my own.
Today I went searching online and found this idea from Tom Hay.
Basically two cross-connected differential amps with the noninverting
input &#2013266065;ground&#2013266066; coming from the output of the other opamp.
Slick!  (LT spice file attached)

Any other ideas on how to do this?
Maybe there is already an IC that does this?
</QUOTE>


How about  MC1496 or AD633? 


On Thu, 10 Nov 2011 18:43:08 +0000, Andrew Holme wrote:

> "George Herold" <gherold@teachspin.com> wrote > > <QUOTE> > I&#146;ve been mucking about with double balanced mixers. I wanted something > to work below 100kHz, so I ordered a few audio transformers. They > worked fine, but it seemed to me that you should be able to replace the > transformers with some opamps. I bashed my head against the wall* for a > day or so trying to come up with a circuit of my own. Today I went > searching online and found this idea from Tom Hay. Basically two > cross-connected differential amps with the noninverting input &#145;ground&#146; > coming from the output of the other opamp. Slick! (LT spice file > attached) > > Any other ideas on how to do this? > Maybe there is already an IC that does this? </QUOTE> > > > How about MC1496 or AD633?
Or NE612. -- www.wescottdesign.com
On Thu, 10 Nov 2011 09:04:45 -0800, George Herold wrote:

> I&rsquo;ve been mucking about with double balanced mixers. I wanted something > to work below 100kHz, so I ordered a few audio transformers. They > worked fine, but it seemed to me that you should be able to replace the > transformers with some opamps. I bashed my head against the wall* for a > day or so trying to come up with a circuit of my own. Today I went > searching online and found this idea from Tom Hay. Basically two > cross-connected differential amps with the noninverting input &lsquo;ground&rsquo; > coming from the output of the other opamp. Slick! (LT spice file > attached) > > Any other ideas on how to do this? > Maybe there is already an IC that does this? > > George H. > > *Sometimes I find something, but most of the time I just put a dent in > the wall. >
It seems overly complicated. What are you trying to do? What are your real frequencies and signal levels? You do realize that with a diode- ring mixer you want the diodes to be either ON or OFF -- no namby-pamby in-between stuff. This means that the LO input can be driven with a square wave -- and a square wave means you can drive that input with logic. And if you _can't_ accept driving the LO input with a square wave, then you need to rethink the whole "mixer" concept and ask yourself if you should be using a real multiplier, instead. -- www.wescottdesign.com
On Nov 10, 1:43=A0pm, "Andrew Holme" <a...@nospam.com> wrote:
> "George Herold" <gher...@teachspin.com> wrote > > <QUOTE> > I=92ve been mucking about with double balanced mixers. > I wanted something to work below 100kHz, so I ordered > a few audio transformers. =A0They worked fine, but it seemed > to me that you should be able to replace the transformers > with some opamps. =A0I bashed my head against the wall* for a > day or so trying to come up with a circuit of my own. > Today I went searching online and found this idea from Tom Hay. > Basically two cross-connected differential amps with the noninverting > input =91ground=92 coming from the output of the other opamp. > Slick! =A0(LT spice file attached) > > Any other ideas on how to do this? > Maybe there is already an IC that does this? > </QUOTE> > > How about =A0MC1496 or AD633?
Ah, Thanks Andrew. I was not so much interested in the mixing aspects of the circuit. But how to make a transformer out of opamps. Or how to make a floating voltage source. (Of course with the opamp it can only float up to it=92s rails.) Here=92s the cross connected differential circuit. GND---RRR---+---RRR--+ | | | |\ | +-|-\ | | >---+-R1--+----(+) Out +-|+/ | | |/ | +-----RRR--+----RRR=97------)---+ | | | Vin-+ GND=97RRR---+---RRR-----+ | | | | | | |\ | | +-|+\ | | | >---+-R1---+-- (-) Out | +-|-/ | | | |/ | +----RRR------+---RRR--+ RRR=92s are like 10k and R1 10 ohms. George H.
On Nov 10, 1:58=A0pm, Tim Wescott <t...@seemywebsite.com> wrote:
> On Thu, 10 Nov 2011 09:04:45 -0800, George Herold wrote: > > I=92ve been mucking about with double balanced mixers. I wanted somethi=
ng
> > to work below 100kHz, so I ordered a few audio transformers. =A0They > > worked fine, but it seemed to me that you should be able to replace the > > transformers with some opamps. =A0I bashed my head against the wall* fo=
r a
> > day or so trying to come up with a circuit of my own. Today I went > > searching online and found this idea from Tom Hay. Basically two > > cross-connected differential amps with the noninverting input =91ground=
=92
> > coming from the output of the other opamp. Slick! =A0(LT spice file > > attached) > > > Any other ideas on how to do this? > > Maybe there is already an IC that does this? > > > George H. > > > *Sometimes I find something, but most of the time I just put a dent in > > the wall. > > It seems overly complicated. =A0What are you trying to do? =A0What are yo=
ur
> real frequencies and signal levels? =A0You do realize that with a diode- > ring mixer you want the diodes to be either ON or OFF -- no namby-pamby > in-between stuff. =A0This means that the LO input can be driven with a > square wave -- and a square wave means you can drive that input with > logic. > > And if you _can't_ accept driving the LO input with a square wave, then > you need to rethink the whole "mixer" concept and ask yourself if you > should be using a real multiplier, instead. > > --www.wescottdesign.com- Hide quoted text - > > - Show quoted text -
Hi Tim, Thanks. I 'think' the idea is to contrast diode mixers with other means of mixing signals. So there will also be an analog multiplier as part of the 'thing'. I'm afraid to say more about the 'thing' cause it's still a bit nebulous and I'm waiting for the various parts to congeal into some coherent whole. The person pushing this project is coming into town this weekend so I=92ll have a clearer view of where we are going after that. I just got intrigued on my own by the transformer to opamp idea. We could then have a diode mixer that operated from DC to ~1MHz. Of course the whole idea might end up in the trash can. George H.
On Thursday, November 10, 2011 9:04:45 AM UTC-8, George Herold wrote:
> ... I ordered > a few audio transformers. They worked fine, but it seemed > to me that you should be able to replace the transformers > with some opamps.
If it's just to take a differential input voltage and make a level-translated (floating) output voltage, there's the OTA (operational transconductance amplifier, like LM13700). For inputs in the few-to-50 mV range, it produces a proportional output current, that you can return through a resistor to any low-Z point. A true transformer produces two matched output currents, one positive and one negative, and you can center-tap it, so it's hard to see how an op amp trick can really do a good emulation. If you were to wire the two OTA sections in opposite input polarities, it would come close. The same LM13700 can be wired to make a multiplier, though; it wouldn't generally do mixer duty by transformer emulation.
On Nov 10, 12:25=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Thu, 10 Nov 2011 09:04:45 -0800 (PST), George Herold > > > > > > <gher...@teachspin.com> wrote: > >I=92ve been mucking about with double balanced mixers. > >I wanted something to work below 100kHz, so I ordered > >a few audio transformers. =A0They worked fine, but it seemed > >to me that you should be able to replace the transformers > >with some opamps. =A0I bashed my head against the wall* for a > >day or so trying to come up with a circuit of my own. > >Today I went searching online and found this idea from Tom Hay. > >Basically two cross-connected differential amps with the noninverting > >input =91ground=92 coming from the output of the other opamp. > >Slick! =A0(LT spice file attached) > > >Any other ideas on how to do this? > >Maybe there is already an IC that does this? > > >George H. > > There's the simple +1/-1 opamp thing, discussed here before. Just a > few parts. The only gotcha is that the LO is essentially a square > wave. > > John- Hide quoted text - > > - Show quoted text -
I had this epiphany driving in to work today, when I realized that a double balanced mixer was just like a +1/-1 switching amp. Did I mention I am slow*? Thanks to all you others with similar hints, Jamie, Tim, Andrew, etc... Any clues for finding the +1/-1 opamp thread? (The diodes like a square wave drive too, as Tim said.) At what frequency do diodes start to beat out Fet switches these days? George H. *Until at least age 20, I was still connecting words between my two separate spoken and written vocabularies.
On Nov 12, 12:03=A0pm, whit3rd <whit...@gmail.com> wrote:
> On Thursday, November 10, 2011 9:04:45 AM UTC-8, George Herold wrote: > > ... I ordered > > a few audio transformers. =A0They worked fine, but it seemed > > to me that you should be able to replace the transformers > > with some opamps. > > If it's just to take a differential input voltage and make a > level-translated (floating) output voltage, there's the OTA > (operational transconductance amplifier, like LM13700). > For inputs in the few-to-50 mV range, it produces a > proportional output current, that you can return through > a resistor to any low-Z point. =A0A true transformer produces > two matched output currents, one positive and one > negative, and you can center-tap it, so it's hard to see > how an op amp trick can really do a good emulation. > If you were to wire the two OTA sections in opposite > input polarities, it would come close. > > The same LM13700 can be wired to make a multiplier, though; > it wouldn't generally do mixer duty by transformer emulation.
Thanks Whit3rd, I've never used an OTA. A colleague reminded me today that Instrument amps have a floating output. I think we should do a switching amp thing. The easy solution is to plug in an AD630 that we already use elsewhere. George H.