Hi all

I want to use npn (BF199) to convert the sin wave from my oscillator to a digital wave ( of 2V PK-Pk).

At the moment I am getting something close to digital wave but with 1.6 V P-P.

The base of the npn has a sin wave at 124 KHz with 800mV p-p. The collector is connected to a voltage divider. The emitter goes to ground.

Any ideas as to how I can make my wave look square and increase the Vp-p .

Thanks

 

A Schmidt trigger comes to mind. You can buy them in a chip or build them with transistors.

There may be reasons why this is impractical but you didn't include any of those reasons in your question.

 

Is there a way I can fix my design ?

 

There are a dozen ways, but you aren't saying anything like how many transistors are available, what your DC voltage is, if you have a capacitor to AC couple the almost kind of sine looking wave, if you have pnp transistors, how important the rise time is, whether you can get or use IC's, etc.

Give me some clues!

 

Why do you need R1 and R3 in that circuit?

 

Here are my circuit characteristics:
1) Using 9V DC Power supply
2) Built a colpitts oscillator that oscillate at 124 KHz ( Very low Peak-peak though, about 500mV)

3) This signal ( sine wave at 124KHz at 500mV p-p) is connected to the base of the transistor

4) The transistor Im using is NPN (BF1999)

5) In the end, all I want is that sine wave to turn digital with 2-3V P-P so that I can use a PIC to read the frequency

 

Why do you need R1 and R3 in that circuit?

Would it work without them? R1 connects emitter down to ground and R3 and R2 maintains collector volatge

 

Omit R3 and replace R1 with a short to GND.

Show us the circuit of the colpitts oscillator.

 

If you disconnect the transistor from the voltage divider, what voltage would you expect at your "output" junction with 9V at the top. Check the values of the voltage divider resistors.

 

If you disconnect the transistor from the voltage divider, what voltage would you expect at your "output" junction with 9V at the top. Check the values of the voltage divider resistors.

To be honest Im not sure what voltage to expect. The sine wave that Im feeding the base of the transistor is at very low volatge - about 800mV.

The positive part of the sine wave turns the transistor on and the negative part turns the transistor off. This phenomena should give me a nice proper digital wave.

 

Omit R3 and replace R1 with a short to GND.

Show us the circuit of the colpitts oscillator.

I just tried this, It gives me this ( please see the attached pic). The Peak to peak volatge is 8.5 V ( total supply volatge) .

Is there a way I can modulate the Vout to 3V ? I cant feed the PIC 8V.

 

Put the top of R2 to the VDD of the PIC instead of 9V.

 

Put the top of R2 to the VDD of the PIC instead of 9V.

Thanks will try that

 

One last question, can the signal be more even ? Can I get a perfect TTL with same ON and off time. Currently the output signal looks like this :

 

Heres the current output

 

.................

 

It would be difficult to get exactly 50% duty cycle. If you are feeding this into the input of a PIC then it should not really matter.

You can try increasing the value of the pullup resistor.