[sdiy] simple transistor buffers?

[Speth, John]

To continue Harry's thoughts,

When the tranny is configured as an emitter follower, the emitter will always be .7V less than the base voltage.  The BE junction must always be forward biased or the emitter voltage ceases to "follow" the input.  The base resistor limits the current through the junction.  The ensure the junction is always forward biased even with negative input voltages, you should connect the emitter to -v through a resistor.  The exact resistor values will depend on the transistor you pick and your supply and input voltages.  I've found typical trannies will give you a wide leeway on resistor values for audio frequencies.

If you AC couple the input, I think you'll need to bias the base voltage so that it won't float off to one of the rails (not sure which one).  There's not much point to AC coupling the input without AC coupling the output unless you find the DC offset on the output desirable.

The key to remember is that the transistor must be properly biased (operate within the "linear" region) all the time when it functions as an amplifier.

If I were you, I'd go with a FET and not a BJT.  I think an FET is a little simpler to apply.

John Speth
Coherent Molectron
mailto:john.speth at coherent.com


> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Harry Bissell
> Sent: Friday, November 19, 2010 8:08 AM
> To: dan snazelle
> Cc: synth-diy DIY
> Subject: Re: [sdiy] simple transistor buffers?
>
> emitter follower always has about a .7V drop at the emitter
> (the b-e drop).  Its good for increasing current but does not
> preserve voltage levels.
>
> Some J-fet followers do a bit better than this...
>
> The op-amp follower is the standard if you need precise voltages.
>
>
>
> ----- Original Message -----
> From: dan snazelle <subjectivity at hotmail.com>
> To: synth-diy DIY <synth-diy at dropmix.xs4all.nl>
> Sent: Fri, 19 Nov 2010 11:02:04 -0500 (EST)
> Subject: [sdiy] simple transistor buffers?
>
> So i am working on a circuit where there are a bunch of outputs.
> to save space/opamps I decided it would be nice to use transistor
> buffers (emitter followers)
>
> however when i switched to what i THOUGHT was a usable buffer, the
> output gain (versus opamp followers) dropped drastically.
>
> let me see if i understand this properly. ( i dont think i do)
>
> if i want to buffer an lfo that is running between + and - 12
> volts, I was putting the oscillator wave through a 1k resistor
> into BASE. I was then putting the collector to +v. What confuses
> me is what to do with the emitter other than
> this is where i take the output and a resistor to ground or -v
> should connect here.
>
> I am not sure if i should put emitter to GROUND or to -v through
> say a 10k resistor.
>
>
> Also, do I need to put a cap before the input to the base?
>
>
> any help and or links to schematics for reference highly
> appreciated
>
>
>
>
> thanks!
>
>
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
> --
> Harry Bissell & Nora Abdullah 4eva
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy

The information contained in this communication is confidential and may be legally privileged. It is intended solely for the use of the individual or entity to whom it is addressed and others authorized to receive it. If you are not the intended recipient you are hereby (a) notified that any disclosure, copying, distribution or taking any action with respect to the content of this information is strictly prohibited and may be unlawful, and (b) kindly requested to inform the sender immediately and delete any copies. Coherent is neither liable for the proper and complete transmission of the information contained in this communication nor for any delay in its receipt.