hi thor,
You cannot compare the BJT Astable oscillator with OPA Triang/Square generator.

Its like comparing an apple and orange and asking which is the 'best'.

E

So what should i do?

 

The circuit with two BJT is not a VCO. It is simply an oscillator (astable multivibrator). There no use to try changing its frequency using an external voltage Vin. The oscillation frequency is given by 1/(0.69*R1*C1). See https://en.wikipedia.org/wiki/Multivibrator

Varying Vin in that circuit, which supplies only the base resistors, will change the frequency as it will change the current charging the capacitors.

 

hi thor,
Do you want me to check your circuit LTSpice simulation of these two circuits.?

If yes, post the LTSpice asc files and I will be happy to check them out for you.

E

 

hi thor,
Do you want me to check your circuit LTSpice simulation of these two circuits.?

If yes, post the LTSpice asc files and I will be happy to check them out for you.

E

Thank you so much.

 

hi,
Downloaded your asc files, will post back later.
E

 

hi,
The BJT Astable does not work correctly when driven by the Triang waveform.
All that happens is the Collector of BOTH transistors switch at the same time, no oscillation of the Astable, see image.

E

 

hi,
The OPA circuit, as posted, does not work correctly when driven by the Triang waveform.

The Triang modulation voltage cannot be allowed to swing to zero, how can it enable the osc with zero voltage drive.??

Limiting the lower excursion of the triang control voltage to +1.5v [not 0v] and a maximum of +7v, shows some level of Frequency modulation.

E

 

hi,
The OPA circuit, as posted, does not work correctly when driven by the Triang waveform.

The Triang modulation voltage cannot be allowed to swing to zero, how can it enable the osc with zero voltage drive.??

Limiting the lower excursion of the triang control voltage to +1.5v [not 0v] and a maximum of +7v, shows some level of Frequency modulation.

E

How can we fix the circuit ?