tactics for increasing charge /discharge of a system

Thread Starter

yef smith

Joined Aug 2, 2020
806
Hello,I have the following circuit where my output is on the buttom call out net.
as you can see my out pulse has a very big charge and discharge time.
Is there a way to make the out pulse charge and discharge faster.
One way i thought is to use PNP or mosfet with smaller capacitances .
Or to use some sort of diode trick.
Is there some practical thing that could be done?
Thanks.

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MisterBill2

Joined Jan 23, 2018
18,928
First, is this an actual physical circuit, or just a simulation circuit?? Next, have you done a plot of the base current and the base voltage??
Why two different transistors in parallel?? And the 2N2907 is a general purpose medium power AUDIO transistor So you are simply not going to get super fast switching in an audio transistor..
 

Thread Starter

yef smith

Joined Aug 2, 2020
806
Hello MisterBill2, given the capcitance model could you give me mathematical intuition regarding why adding this capacitor drasticly reduces the charge and discharge of the model?

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MisterBill2

Joined Jan 23, 2018
18,928
Short answer: no.
I have been a believer in accepting data sheet claims as to response and performance of components for many years and while I suppose I did use those formulas and equations in that class, I have not needed to use them again since 1968.
The fact is that some transistors are much faster and others are not.
And going by the data sheets, correctly understood, has not let me down so far. Of course, understanding what they say has, on occasion, required conversations with various application assistance folks. Those folks are usually able to provide the needed answers.
 

crutschow

Joined Mar 14, 2008
34,676
Below is the sim using a single high-speed transistor designed for switching:
Why did you have two?
It shows about a factor of 10 reduction in rise-time

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Thread Starter

yef smith

Joined Aug 2, 2020
806
Hello Papabravo,I have tried to put capacitor in parralel to R2 with various values but the rise time is still much better then the fall time.
I know i can remove one transistor or wsitch into another bjt.
Is there some other method to improve the fall time?
Thanks.

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You can also put a speedup cap across R2 to improve the turnoff time, if that was your only problem.
 

Papabravo

Joined Feb 24, 2006
21,256
Hello Papabravo,I have tried to put capacitor in parralel to R2 with various values but the rise time is still much better then the fall time.
I know i can remove one transistor or wsitch into another bjt.
Is there some other method to improve the fall time?
Thanks.

View attachment 319636
So, the transistor is ON when the input is at -5.2V and it turns OFF when the input rises to -3V. The speedup capacitor on R1 did the expected job of reducing the TURN OFF time. Things are backwards with PNP and a negative supply.
 

Papabravo

Joined Feb 24, 2006
21,256
Hello crutschow, given the data sheet below could you tell why 2N5771 is better then 2SB1695
Could you show what is the charge and discharge equivalent model capcitance so i could try and simulate and see the difference?
Thanks.
https://www.mouser.co.il/datasheet/2/308/2N5771-1118378.pdf
https://datasheetspdf.com/pdf-down/2/S/B/2SB1695_Rohm.pdf
The only relevant parameter that occurs in both datasheets is Cob. The 2SB has about 4 times the value of the 2N5771. They both appear to have a transition frequency in the 250-300 MHz. range.
 
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