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Transistor switching

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Murugesh_89

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In the following circuit,

Transistor switching problem .jpg

Supply Voltage at the collector 22k resistor is 5V.

when i input the R1 with the 5v then i get the output as Q1C,V = 0.03V as i expected.

But when i input the R1 with 0V then i did not get the complete 5V at the Q1C instead i am getting 4.15V. I tried with R1 as 1K and R2 as 4.7K then also same problem. This results i am getting in simulation.

When i did it practically i am getting 2V at the Q1C.

When i removed the connection to the mux from transistor then it is working perfectly.

Why it is happening like this when i interface with mux. How do i solve this problem?

Thanks,
Murugesh
 

Because that mux is loading down the output, try a 220k across the output and the connection to the mux
 

Loading down means by what value of resistor? Do you have any internal schematic of 74151.
 

Its pretty much voltage divider law. If the output voltage dropped to 2V then the input resistance of the 74151 mus be around 15k, hence Vout = R2/(R1 + R2) * Vin
Since your collector resistor is 22k, and you dont want the voltage to drop so you must set your output resistance to be 220k or higher
 

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