Variable offset pulsing circuit-Transistor connections?

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electronice123

Joined Oct 10, 2008
311
Hi all,

I am working on a circuit that is supposed to produce a train of square waves at one amplitude then at a lower amplitude and switch back and forth from high amplitude pulses to low amplitude pulses. See schematic/waveforms

I designed the circuit in multisim and it worked great the first time....But in real life its not working right-I think my problem is because I'm using a different voltage across the transistors so the base current is getting distorted since it comes from a different circuit and is at a different voltage???

WhenI remove Vee in multisim and ground that point to the other circuit I get exactly what I need.....But in real life when I ground the lower transistor to the supply ground the amplitude across the load drops to nothing.
VVPS.png
I'm using a dual variable power supply and a dual channel signal generator....Signals look exactly as they are supposed to until I look across the 10 ohm load??

Thanks
 

AnalogKid

Joined Aug 1, 2013
8,140
You're putting a 12 V p-p waveform into an inverting amplifier powered by only +12 V. Even if the input common mode range included both power rails, the only DC offset that would produce a clean output is 12 V. Any adjustment of the offset pot and the opamp output will saturate against one rail.

Neither of the waveform drawings have voltage levels or a ground reference, so it is not clear at all what is going on.

Also, the "what I need" waveform looks like it never goes below GND, yet you have a TIP120 pulling the load to -5 V. ??? What is the relationship with both 10 ohm resistor terminals and GND? Or, what are the voltages (with respect to GND) for the bottom, lower peak, and higher peak output voltages?

A TIP120 is rated for 5 A and your max output current is 1 A, so there is no need for the 2N3055.

R1 is way too small, allowing almost 60 mA of base current where 2 mA is sufficient. Consider 1.0 K - 2.2 K.

ak
 

Thread Starter

electronice123

Joined Oct 10, 2008
311
Thanks...I'll make the changes you suggested and test the circuit again.

I forgot to mention the square waves are 12V with a 6V offset so they never go below 0V
 
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