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4 transistor discrete opamp (LTspice problem or circuit problem?)

Why doesn't this discrete 4T opamp run in LTspice at higher frequencies? It looks ok at 1Khz, but I get a 5 meg oscillation at 10K or more. There are no oscillations with the input disconnected.

Version 4 SHEET 1 880 680 WIRE 128 -112 32 -112 WIRE 192 -112 128 -112 WIRE 400 -112 192 -112 WIRE 512 -112 400 -112 WIRE 400 -96 400 -112 WIRE 32 -64 32 -112 WIRE 128 -16 128 -32 WIRE 128 -16 96 -16 WIRE 192 -16 192 -32 WIRE 304 -16 192 -16 WIRE 400 16 400 -16 WIRE 576 16 400 16 WIRE 640 16 576 16 WIRE 400 32 400 16 WIRE 32 80 32 32 WIRE 304 80 304 64 WIRE 304 80 32 80 WIRE 336 80 304 80 WIRE 128 112 128 -16 WIRE 192 112 192 -16 WIRE 512 112 480 112 WIRE 304 128 304 80 WIRE 480 144 480 112 WIRE -64 160 -96 160 WIRE 48 160 16 160 WIRE 64 160 48 160 WIRE 512 192 512 112 WIRE 160 208 128 208 WIRE 192 208 160 208 WIRE 256 208 256 160 WIRE 160 224 160 208 WIRE 304 304 304 208 WIRE 304 304 160 304 WIRE 400 304 400 128 WIRE 400 304 304 304 WIRE 512 304 512 272 WIRE 512 304 400 304 WIRE 48 352 48 160 WIRE 272 352 48 352 WIRE 576 352 576 16 WIRE 576 352 352 352 FLAG 512 -32 0 FLAG -96 240 0 FLAG 256 208 0 FLAG 480 144 0 FLAG 640 96 0 SYMBOL voltage 512 -128 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 39 60 Left 0 WINDOW 3 40 92 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -96 144 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 -77 57 Left 0 WINDOW 3 -110 143 Left 0 SYMATTR InstName V2 SYMATTR Value SINE(0 .5 1k 0 0 0 10) SYMBOL npn 64 112 R0 WINDOW 3 -1 119 Left 0 SYMATTR Value 2N3904 SYMATTR InstName Q2 SYMBOL npn 256 112 M0 WINDOW 0 40 33 Left 0 SYMATTR InstName Q3 SYMATTR Value 2N3904 SYMBOL res 144 208 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL res 32 144 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 1k SYMBOL res 368 336 R90 WINDOW 0 60 116 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 10k SYMBOL npn 336 32 R0 WINDOW 3 77 31 Left 0 WINDOW 0 76 59 Left 0 SYMATTR Value 2N3904 SYMATTR InstName Q5 SYMBOL res 112 -128 R0 WINDOW 0 29 88 Left 0 WINDOW 3 -45 47 Left 0 SYMATTR InstName R3 SYMATTR Value 470 SYMBOL pnp 96 32 R180 WINDOW 0 84 13 Left 0 WINDOW 3 35 44 Left 0 SYMATTR InstName Q4 SYMATTR Value 2N3906 SYMBOL res 176 -128 R0 WINDOW 0 43 82 Left 0 WINDOW 3 39 45 Left 0 SYMATTR InstName R2 SYMATTR Value 470 SYMBOL res 288 112 R0 SYMATTR InstName R8 SYMATTR Value 360 SYMBOL res 384 -112 R0 WINDOW 0 -52 65 Left 0 WINDOW 3 -38 29 Left 0 SYMATTR InstName R11 SYMATTR Value 1k SYMBOL voltage 512 176 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 -71 36 Left 0 WINDOW 3 -67 71 Left 0 SYMATTR InstName V3 SYMATTR Value 15 SYMBOL res 288 -32 R0 WINDOW 0 -44 50 Left 0 WINDOW 3 38 54 Left 0 SYMATTR InstName R6 SYMATTR Value 12k SYMBOL res 624 0 R0 SYMATTR InstName R7 SYMATTR Value 1k TEXT -152 328 Left 0 !.tran .01s

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Reply to
Bill Bowden
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Technically you got the noninverting input grounded and are using the inverting. sopmething has to stop it. You don't have a cap to do that.

Look up "Miller capacitor" and hopefuly that (WIKI maybe) should explain it. If not, someone around here should be articulate enough.

Reply to
jurb6006

[Full listing at Message-ID: ]

See attachment.

0dB, it's just fine ;-)

LTspice, nor most any other Spice for that matter, will not show oscillations in a static .TRAN analysis unless you push start them.

A .AC analysis of the Loop Gain and Phase shows that it fails the criteria for stability.

What are you attempting to do? Or is this just a self-educational exercise?

Suggestion: Go back and do your "disconnected" situation, but ramp up the supplies from zero... then it'll (most likely) show oscillations. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food. 

begin 644 OpAmp_Bowden.pdf 
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M;V)J#3(@,"!O8FH\/"]3+T0^/@UE;F1O8FH-,R`P(&]B:CP\+T-O=6YT(#(O 
M5'EP92]086=E];DY_DS[Z=PVS$_T4U>V)'TX:3OQU7XM*I(,G$A(K`L4]7X4U9%%,W4H.+J=(Q8(\VKA 
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M*GCFVD:9V%IL>7KWK4KFE],CR=,^Q'T0\)T[5.]DN*K>J%2_85S`/4V7GU;U 
M2W:\57%%N,YD5]CY[%ZN""MI:@U>\-QJE:T6:IOV^3B2DWU^-%PZ*9LWHAV$ 
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Reply to
Jim Thompson

[Full listing at Message-ID: ]

Looks like S.E.B doesn't allow attachments, so see...

0dB, it's just fine ;-)

LTspice, nor most any other Spice for that matter, will not show oscillations in a static .TRAN analysis unless you push start them.

A .AC analysis of the Loop Gain and Phase shows that it fails the criteria for stability.

What are you attempting to do? Or is this just a self-educational exercise?

Suggestion: Go back and do your "disconnected" situation, but ramp up the supplies from zero... then it'll (most likely) show oscillations. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

It's just self-educational.. The DC imbalance Q2/Q3 did seem to be a problem as you say. Reducing R3 to 82 ohms seemed to cure the oscillation problem. I was attempting to create an opamp with the fewest transistors. I thought the minimum might be 4, but after looking at it, I didn't see a need for the PNP transistor and did it again using just 3 NPNs (Q4 and R3 are gone). And I think there is a way using just 2 transistors, but it wouldn't have both inverting and non-inverting inputs. I don't have much experience with LTspice, the only AC analysis I've done is sweeping from frequency A to B.

-Bill

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Reply to
Bill Bowden

You need a capacitor in there to keep it stable under all feedback conditions.

I'll modify your basic structure with a few gimmicks I've learned after more than 50 years of designing OpAmps... maybe tomorrow if I have enough time... I have a design review tentatively scheduled. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

That reduced the open loop gain of the ciruit.

Reply to
jurb6006

Try this...

...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

That's an interesting implementation of Q2. Pretty sure I've not seen it done that way before. what advantage does that have rsather than the standard collector loaded current source ?

Reply to
jurb6006

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Interesting gimmick using the Vbe drop of Q5 across the 1k resistor to set the current constant at 0.7/1k = 700 uA. The collector just dumps any extra current passing through R8. I was wondering how to keep the current constant with lower suppy voltages. The way I had it would only work at +/- 15 volts. Nice job. How did you arrive at 10pF for the capacitor?

-Bill

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Reply to
Bill Bowden

10pF capacitor determined by running...

...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

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