Transistor switching (Vcc to saturation) at 1MHz

Thread Starter

Zeeus

Joined Apr 17, 2019
615
Trying to switch a transistor on and off (the basic circuit below) at 1 MHz... The output (collector) is like RC charging. Using pn2222 and 2n4124

Need output to be exactly square like input (inverse). What resource to study on this?

Tried baker clamp (with 3 diodes). Improvement but not exactly square

Thanks

1613585846860.png
 

crutschow

Joined Mar 14, 2008
27,198
What transistor are you using?
To minimize saturation turn-off delay you need a transistor designed for fast switching, such as a 2N2369.
Below is the LTspice switching comparison of the three transistors.

You can also reduce the switching time adding a small capacitor across Rb (a couple hundred pF, experiment for the best value).

1613593705830.png
 

crutschow

Joined Mar 14, 2008
27,198
Here's the simulation for a 2N4124 with an input capacitor.
The sim only required about 50pF to eliminate the saturation delay.
Note the large turn-off saturation delay with no capacitor (yellow trace).

The signal source must have a low impedance for the capacitor to work.

1613595099760.png
 

Thread Starter

Zeeus

Joined Apr 17, 2019
615
Switching at 1MHz might be a problem without an active pull up device. What voltage is Vin?
active pulll up? how?

My test circuit vin was 5v... Main circuit is similar to below
1613600891751.png

Question : The output of A1 is positive. When TR1 turns on, there is voltage drop across R7, where does the current flow?

missing something?

Thank you
 

dl324

Joined Mar 30, 2015
12,871
active pulll up? how?
I'd use a CMOS inverter. Integrated if they'll provide sufficient current and have the proper threshold voltage, or discrete.

You could use complementary BJTs, but then you have the same problem with needing speedup caps.

EDIT: removed redundant wording
 
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