I run across the schematic shown in page 13 of the RC4136's datasheet (attached). After 3 days trying to have it "working" in LTSpice (circuit attached) I realized I need help.
I tend to think I know the basics of log / antilog amps and I understand this circuit should give (hopefully) a Vout = Vy * Vx / Vz. Instead, the prevailing result I get is Vout slightly above Vx, no matter what Vy or Vz are.
With different op amps, or no matter how many diodes I pile up at the top (to get up to around 3.5V) to polarize the first stages, I always get a result not even close to the expected (and most of the times with >1 MHz noise riding the output).
I am puzzled by what is wrong: my simulation or the circuit?
If anyone is tempted to tell about the so many pitfalls of a circuit like that implemented with discretes or how old / lousy the RC4136 is, PLEASE don't. I am asking about a simulation and / or eventual flaw of a design.
Helps is appreciated.
I tend to think I know the basics of log / antilog amps and I understand this circuit should give (hopefully) a Vout = Vy * Vx / Vz. Instead, the prevailing result I get is Vout slightly above Vx, no matter what Vy or Vz are.
With different op amps, or no matter how many diodes I pile up at the top (to get up to around 3.5V) to polarize the first stages, I always get a result not even close to the expected (and most of the times with >1 MHz noise riding the output).
I am puzzled by what is wrong: my simulation or the circuit?
If anyone is tempted to tell about the so many pitfalls of a circuit like that implemented with discretes or how old / lousy the RC4136 is, PLEASE don't. I am asking about a simulation and / or eventual flaw of a design.
Helps is appreciated.
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