# Circuit that uses a BJT and 2 opamps

1. Mar 22, 2010

### nand_1

Hi all,

I was reading up on VCO's and found an example circuit that uses a BJT and 2 opamps. I have simulated the circuit on PSPICE and it seems to be oscillating quite nicely, with oscillation frequency in proportion to the input voltage.

The following is the circuit and output (V5 is input voltage and output is were it's marked 'V' with an arrow):
http://img232.imageshack.us/img232/3088/vco.png [Broken]

My issue with this is how do i tune it to map certain voltages with certain frequencies? Apart from playing around with the RC values randomly, is there another way to determine the frequency - voltage relationship mathematically?

I have so far come up with the following formula:

f = Vin / (2 * pi * c * (r4 + r5) * somefactor )

But i can't seen go anywhere with this. For small voltages (in range of 1v to 4v) that formula seems to hold, but any higher input voltages (5-10v), "somefactor" value begins to change significantly.

Any ideas or tips?

Thanks.

Last edited by a moderator: May 4, 2017