will this fsk cicuit work?

[vikas1988]

hi guys. i found this fsk circuit in some old notes. is the design of this circuit correct? can a transistor be used as a switch as in this circuit?

 

[FvM]

You have been drawing a simulation circuit. Why don't you finish the simulation before asking others about the expectable result? Is it quiz?

A bipolar transistor can be used as an analog switch, but in the shown circuit, the input amplitude would be limited to 0.5 or 1 Vpeak.
Try to find out why by observing the waveforms at the invidual circuit nodes and thinking about basic BJT properties.

 

[vikas1988]

i did run the simulation and it worked. i want to know is it possible to connect a sinusoidal volatge source to the emitter without giving any dc bias.

and its not a homework problem

 

[betwixt]

All this schematic does is use one or the other transistor as a clamp across the signal. It is incapable of reducing the unwanted signal to less then VCEsat of the transistor which is conducting. If you try to inject a signal at the emitter by lifting them from ground, all you achieve is a failure to clamp the unwanted input.

You would do better to use a simple analog switch.

Brian.

 

[FvM]

i did run the simulation and it worked

The collector voltage is clamped at about 1.4V, when an inverse collector voltage is applied. But the transistors can fairly work as a ground referenced switch (with reduced magnitude). It doesn't work reasonable as floating switch.