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
Voltage buffer
Apr 14, 2007
5 Replies
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.
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
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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
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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
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).
Wim Lewis , Seattle, WA, USA. PGP keyID 27F772C1
I'm assuming there is a reason you cant use a voltage divider if the transistor is not rated high enough?? B
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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