# What wil be the output?

Discussion in 'General Electronics Discussion' started by vinit2100, Oct 21, 2013.

1. ### vinit2100

100
1
Oct 21, 2013
Transistor switching circuit help !!!

What will be the output VPP_4 when the signal F_6P5_CON_4 becomes high/low (3.3/0) ?

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Last edited: Oct 21, 2013
2. ### Harald KappModeratorModerator

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Nov 17, 2011
This looks like homework, is it?
In that case, I'll move the thread to the homework section.
Show us what you have done so far to come to a solution. We'll help you solve the question yourself, We won't give the answer directly.

3. ### vinit2100

100
1
Oct 21, 2013
I want to know how this circuit works? or what is the purpose of this circuit?

4. ### duke37

5,364
771
Jan 9, 2011
What is the voltage on the base of Q11 when the input is 3.3V and 0V?

Similarly, what is the voltage on the gate of Q12 when Q11 is on or off? Will this turn Q12 on or off?

5. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
This seems to be a switching circuit to switch a 6.5V DC rail onto a signal called VPP_4 which probably connects to a pin on a programmable device that requires 6.5V as a VPP (programming voltage) supply while it is being reprogrammed. This would be used as part of a device or (E)EPROM programmer.

The circuit is quite straightforward. When the input voltage is high, the transistor turns ON and pulls its collector to 0V, forward-biasing the P-channel MOSFET and passing the 6.5V supply voltage to the VPP_4 signal.

When the input voltage is low, the P-channel MOSFET is biased OFF and will not affect the voltage on VPP_4, which may then be driven by other circuitry.

R32 and R28 should be deleted. They serve no useful function.

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
R32 is required if the input is something that has an open collector (or open drain) output and is only able to pull the input to ground.

7. ### duke37

5,364
771
Jan 9, 2011
R32 will keep Q11 turned on even if the input is 0V.

This may be the same for Q12 depending on the required voltages to turn it on.

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Very true. It would have to have a much higher value to behave as I suggested.

9. ### KrisBlueNZSadly passed away in 2015

8,393
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Nov 28, 2011
Right. If R32 is needed, it must be connected BEFORE R27, not after.

If the circuit is being controlled by an open collector/drain output, its logic should be reversed, using another inversion stage probably, so that the open collector/drain output is designated active-low. This is standard practice for open-collector outputs.

If the output is designated active-high it should not have a pullup; a pullup will make the circuit feed 6.5V onto VPP_4 in the default (output open) state, which would occur when the driving device is being held reset, if it's an 8051 for example.

10. ### vinit2100

100
1
Oct 21, 2013
what is the purpose of R32?