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Spice help please

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I was given a preamp circuit where inputs were connected together via very inductive pickup that has a fairly high winding resistance. the NI input also goes to ground via a 3.3Meg resistor, and the inverting input also goes to the output via a 3.3Meg resistor paralled with 220pf comp resistor. Opamp used is the OP-07. Looked ugly to me, most especially the ungrounded input inductor.

So,i re-designed the inductor for about a third winding capacitance, and added a center tap. 1000H and 44K resistance per side. Decided to use the LT1001AM to have a bit better overall noise and stability specs and perhaps noise on the low end (below 10Hz).

If i look at the signal at the top of of L1 i do get the same level at the output (up to 20KHz), nevermind the flat band level at -111dB (used crummy drive on purpose).

But what i do not understand is if i use 44Megs instead, the gain seems to be -9dB, not (in ordinary cases) +60dB. And that peaking also stumps me.

Is that noise plot for real? I expect to see some 1/f...

Reply to
Robert Baer
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Between which two points are you measuring the gain?

Do you mean replace all 4 of the 44K with 44Meg?

Don't know. Do you want to know the noise of the output node, or what?

Reply to
John S

One quick thing, the two halves of the (CCL?) coil are seeing different impedances, so because the sources have a significant impedance it's not differential.

Cheers

--
Syd
Reply to
Syd Rumpo

  • From inductor input to output.
  • the FB resistor and the one to ground.
  • Well, yes,what else?
Reply to
Robert Baer

Looks to me like they have the same resistive loading; L2 sees R2 then R4 to ground, and L1 sees R1 and R3 to Vout. The R1 and R2 values are to emulate the coil resistances.

Reply to
Robert Baer

Yes, exactly. R2 + R4 go to a fixed 0V but R1 + R3 go to the output signal which is moving up and down in antiphase, so L1 sees a lower impedance than L2. It's a common mistake.

Connect a power supply and do a transient sim, then you'll see, particularly look at the current through R1 and R2.

Like this...

WIRE -352 192 -352 160 WIRE -144 224 -144 192 WIRE 208 224 208 192 WIRE -352 304 -352 272 WIRE 32 304 32 256 WIRE 64 304 32 304 WIRE 160 304 160 176 WIRE 160 304 144 304 WIRE 192 304 160 304 WIRE 320 304 272 304 WIRE 320 336 320 304 FLAG 64 176 0 FLAG 320 336 0 FLAG -192 160 0 FLAG -144 384 0 FLAG 352 160 Vo FLAG -384 160 0 FLAG -352 32 +9 FLAG 208 96 +9 FLAG -352 304 -9 FLAG 208 224 -9 SYMBOL Opamps\\LT1001A 208 96 R0 SYMATTR InstName U1 SYMBOL ind2 48 160 R180 WINDOW 0 36 80 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName L1 SYMATTR Value 1000 SYMATTR Type ind SYMBOL ind2 48 272 R180 WINDOW 0 36 80 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName L2 SYMATTR Value 1000 SYMATTR Type ind SYMBOL res 48 32 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R1 SYMATTR Value 44k SYMBOL res 160 288 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 44k SYMBOL res 176 32 R270 WINDOW 0 31 82 VTop 2 WINDOW 3 60 20 VBottom 2 SYMATTR InstName R3 SYMATTR Value 44k SYMBOL res 288 288 R90 WINDOW 0 65 30 VBottom 2 WINDOW 3 36 89 VTop 2 SYMATTR InstName R4 SYMATTR Value 44k SYMBOL ind2 -160 96 R0 SYMATTR InstName L3

SYMATTR Type ind SYMBOL voltage -144 288 R0 WINDOW 123 -50 133 Left 2 WINDOW 39 0 0 Left 2 WINDOW 3 24 44 Left 2 SYMATTR Value2 AC 1mV SYMATTR InstName V1 SYMATTR Value SINE(0 1m 10k) SYMBOL res -128 320 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R5 SYMATTR Value 1k SYMBOL voltage -352 48 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 9 SYMBOL voltage -352 176 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value 9 TEXT 16 408 Left 2 !;ac dec 10 0.001 1e6 TEXT -192 48 Left 2 !K L1 L2 L3 1 TEXT -16 440 Left 2 !;noise v(n002) V1 dec 10 0.001 1e6 TEXT -48 352 Left 2 !.tran 1m

Cheers

--
Syd
Reply to
Syd Rumpo

From the dotted end of L3 to Vo? The gain is 90dB for your posted circuit.

With 44M where you describe the gain is about 89dB.

Well, your .noise statement says you want V(n002) not V(Vo).

Reply to
John S

Was this of no help to you?

Reply to
John S

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