I've been working on building a frequency to voltage converter using discrete components (before you suggest an LM331 or similar, I wish to implement such a device using discrete components, not use an IC), for this I've used a low pass filter (because noise is high frequency so I figured I should filter that out) to provide a frequency-dependent transfer function, and implemented it this way:

Frequency to voltage converter using discrete components

So far, It's linear in the frequency range I wish it to, however I have a problem: It reaches a value and then tends to oscillate around that value (reaches 7.41, then 7.42 then 7.38 then 7.41 again, etc), which of course is not behaviour you'd want from such a device.

My question is: How can I minimize or get rid of said oscillations?

Thanks a lot for your help

  • 1
    \$\begingroup\$ Filter harder . \$\endgroup\$ Commented Sep 7, 2015 at 1:13
  • \$\begingroup\$ Please could you explain a little more what you're referring to? \$\endgroup\$
    – HCalderon
    Commented Sep 7, 2015 at 1:57
  • \$\begingroup\$ There are at least two components that serve no purpose at all: D4 and R3; probably also D2. And another one that hurts: D3 \$\endgroup\$
    – Curd
    Commented Feb 10, 2016 at 8:40
  • \$\begingroup\$ You don't want to use ICs. So you built the OpAmps with discrete transistors?! \$\endgroup\$
    – Curd
    Commented Feb 10, 2016 at 8:41

1 Answer 1


I see that D4 is shorted by having both pins to ground.If D1-D4 form a limiter/squarer, the connection is wrong. Take a look at this: http://electronicdesign.com/digital-ics/whats-all-frequency-voltage-converter-stuff-anyhow Plus, I don't agree with U3 directly on C2. I suggest to put a resistor, so it becomes a simple integrator.

  • \$\begingroup\$ Also U1 is directly driving a virtual earth on U2 with no 'input' resistor. This can't end well. \$\endgroup\$
    – Transistor
    Commented Feb 9, 2016 at 19:38

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