# Voltage buffer

Discussion in 'Electronic Basics' started by [email protected], Apr 14, 2007.

1. ### Guest

In a circuit I have DC signal somewhere around 10V-14V. This I would
like to use as input for a load of 13kO. However, the output circuit
should only be minimally loaded. Now if this signal was <6V I would
just use whatever NPN I can find in my drawers and set it up as an
emitter follower, however with the voltage being that large I guess
that would kill the transistor- or would it? Any other simple
solutions?

2. ### Andrew HolmeGuest

Most NPN transistors in my drawers will happily work at that voltage. I
probably don't have anything with VCEO less than 20 or 30V.

3. ### ChrisGuest

The power to drive the output circuit has to come from somewhere else
-- I'm assuming you've got a DC power supply which is greater than the
highest amplitude of your input signal.

If you do, why not just use an opamp as a voltage follower, like this
(view in fixed font or M\$ Notepad):

|
| +24V
| |\|
| >-------|+\
| Vin | >---o----->
| 10-14V.---|-/ | Vout
| | |/| |
| | | |
| | === |
| | GND |
| '----------'
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

An LM741 will work just fine here. Or if your load requires more than
20mA, you can put your transistor in the feedback loop as a current
booster, like this:

|
| +24V +24V
| |\| |
| >-------|+\ |/
| Vin | >------|
| 10-14V.---|-/ |>
| | |/| |
| | | |
| | === |
| | GND | Vout
| '---------------o-------o
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Either way, your output voltage will track the input within a few
millivolts, and your signal source will be unloaded. Much better
solution than a single transistor.

Good luck
Chris

4. ### Wim LewisGuest

Even the common-as-beans PN2222A has a Vceo max of 40 volts, according
to the first datasheet I found. Power dissipation should be under
100 mW, well under the 625 mW that the 2222 claims to be happy
with. So yeah, I think a random junk-drawer transistor has a good
chance of working in this circuit (since all the transistors in my
junk drawer seem to be 2222s or BC547s).

5. ### Brian OGuest

I'm assuming there is a reason you cant use a voltage divider if the
transistor is not rated high enough??
B

6. ### Guest

Okay, this was a basic misunderstanding on my part. I assumed that the
maximum Vbe rating is what I am allowed to apply to the transistor
base. For the above scenario a regular 2N2222 turned out to work just
fine.

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