Hello,

I am trying to make a cheap signal generator with variable frequency, amplitude and rise & fall time smaller than 100ns. I'm using MCU GPIO to produce square wave and DAC to change amplitude. I attach a schematic.

I've built the circuit, but the rise time is 500ns or something like that.I think the pull-up resistor is limiting the signal speed.But when i lower the value of pull-up, DAC is not able to produce required current.

Please share ideas how can i improve the circuit. Thank you

 

Use a pushpull output using an npn/pnp transistors...

 

Use a pushpull output using an npn/pnp transistors...


View attachment 130136

Where should i connect DAC output in this push pull configuration?

 

To the 5-12v terminal, i take it the square wave is a fixed amplitude and the dac is just a variable dc voltage supply?

 

To the 5-12v terminal, i take it the square wave is a fixed amplitude and the dac is just a variable dc voltage supply?

Yes the square wave is fixed ampliture, but the DAC is the internal DAC of microcontroller. It only gives a few mA..

 

Then you need to feed the Dac to the bases, and use the Emitters as the dac output and use another pushpull output for the square wave output, using the dac transistors as the Collector supply for that, the both outputs will be mixed.

 

Then you need to feed the Dac to the bases, and use the Emitters as the dac output and use another pushpull output for the square wave output, using the dac transistors as the Collector supply for that, the both outputs will be mixed.

Did i correctly draw the schematic ? Where do i connect other side of DAC transistors pair (i drew red question mark there)?

 

No,

 

Hello, thanks for the schematic. I simulated it in LTSpice. It works! The mixed output changes from 1.27 to 3.3V.
But there are two problems:

1) I will control the amplitude from MCU in digital steps: for example: 0V..200mV..400mV..600mV. I think it would be hard to predict what DAC value i need to set to get required amplitude.
2) I would like for output amplitude to start from lower voltage for example 0.2V. Now it starts only from 1.27V