Anybody done anything interesting with a common base configuration?

[zachomega]

Just curious if anybody could point me in some directions...

-Zach Omega

[Paul Perry (Frostwave)]

http://www.allaboutcircuits.com/vol_3/chpt_4/7.html

I can't point ot anything 'interesting' - I do remember that it was used in RF amps, think for some reason it had better HF response - but I did use a grounded base configuration once when I only had room for one transistor and a couple of resistors, and needed a non-inverting voltage amplifier (for a gate voltage that had to be cranked from 5 to 10).

[zachomega]

Thanks for the link...I have some reading to do. 

-Zach Omega

http://www.allaboutcircuits.com/vol_3/chpt_4/7.html

I can't point ot anything 'interesting' - I do remember that it was used in RF amps, think for some reason it had better HF response - but I did use a grounded base configuration once when I only had room for one transistor and a couple of resistors, and needed a non-inverting voltage amplifier (for a gate voltage that had to be cranked from 5 to 10).

[DDD]

Pls breadboard or simulate the common base stage as the input stage of the guitar preamp\OD\fuzz. Also pls note that adding various capacitors in parallel with pickup will make absolutely different frequency response compared with the traditional pickup response. Surely you'll find it very interesting to play with that CB BJT stage (I did it a long ago and have been surprised a lot with differnt sounds).
The same effect can be achieved with the "OpAmp inverting stage without the input resistor". To be correct the pickup impedance plays a role of the "input resistor" here.

[Nasse]

Just for fun I did a crude mixer years ago, a very simple circuit with minimum parts, one transistor "common base" circuit. I used to monitor a four track tape recorder with it. I put it in a box. Some idiot stole it from me

[Sir H C]

Hey the common base is part of the differential pair, it is used a lot 

I have done some cascoding using the common base stage off the collector of another stage.  It doesn't really do that much as the output impedance is set by the resistive load more than the device, but you do get some more voltage gain for a "one stage" setup. 

I guess you could also come in to a pedal via the emitter and out the collector, but it would either be very high impedance (you get no current gain in the common base configuration) or require a very low impedance input.

Still cascodes are where the common base comes into their own, that and the output stages of Peavey and Music Man amplifiers with a transistor driving the "grounded grid" (the tube version of common base).

The main use of common base/cascoding is to get rid of the miller effect that slows things down for a common emitter stage.

[zachomega]

I don't suppose you could post a schematic example of what you mean by cascoding the common base stage off of the collector of another stage?  Just a general diagram maybe and a simple explaination of how it works? 

-Zach Omega

Hey the common base is part of the differential pair, it is used a lot 

I have done some cascoding using the common base stage off the collector of another stage.  It doesn't really do that much as the output impedance is set by the resistive load more than the device, but you do get some more voltage gain for a "one stage" setup. 

I guess you could also come in to a pedal via the emitter and out the collector, but it would either be very high impedance (you get no current gain in the common base configuration) or require a very low impedance input.

Still cascodes are where the common base comes into their own, that and the output stages of Peavey and Music Man amplifiers with a transistor driving the "grounded grid" (the tube version of common base).

The main use of common base/cascoding is to get rid of the miller effect that slows things down for a common emitter stage.

[Sir H C]

Okay, imagine you have a standard common emitter stage.  Now where the collector hits the resistor (or current source ) you stick another transistor, emitter to the collector of the one below, and collector to the load.  The base gets a 2 resistor bias from the top rail such that the collector of the bottom transistor is at about .7 to 1 volt (depends on the curves, but usually you go somewhere around there, so to do this you would usually bias it to have about 1.7 to 2 volts on the base).  The top transistor is a really low impedance load for the bottom one so you do not get much voltage swing on the output.  This also means that the output from that transistor is a lot more linear than it would otherwise be as the early voltage effects do not happen much because the swing on the collector is much smaller.  This is used in a lot of circuits to mirror voltages more accurately, especially with sub-micron CMOS which otherwise just sucks.  90nm, blech.

[zachomega]

OKay...I'm vaguely following along...This is making me wish I didn't drop my EE classes in favor of a history degree. 

-Zach Omega

Okay, imagine you have a standard common emitter stage.  Now where the collector hits the resistor (or current source ) you stick another transistor, emitter to the collector of the one below, and collector to the load.  The base gets a 2 resistor bias from the top rail such that the collector of the bottom transistor is at about .7 to 1 volt (depends on the curves, but usually you go somewhere around there, so to do this you would usually bias it to have about 1.7 to 2 volts on the base).  The top transistor is a really low impedance load for the bottom one so you do not get much voltage swing on the output.  This also means that the output from that transistor is a lot more linear than it would otherwise be as the early voltage effects do not happen much because the swing on the collector is much smaller.  This is used in a lot of circuits to mirror voltages more accurately, especially with sub-micron CMOS which otherwise just sucks.  90nm, blech.

[Eb7+9]

common base : a current buffer with a 1/gm impedance looking into the emitter when Base terminal is AC or DC grounded ... you'll find an application in the Oberheim-Maestro S/H by Bob Moog ... namely, the bipolar device interfacing the sampler output to the filter bias ports, which are current mode inputs ...

[Paul Perry (Frostwave)]

common base : a current buffer with a 1/gm impedance looking into the emitter when Base terminal is AC or DC grounded ... you'll find an application in the Oberheim-Maestro S/H by Bob Moog ... namely, the bipolar device interfacing the sampler output to the filter bias ports, which are current mode inputs ...

Nice one, I never really noticed that before! It never ocurred to me that the transistor was actually linear there

[gaussmarkov]

also the boss blues driver bd-2.    http://www.diystompboxes.com/smfforum/index.php?topic=57472.0

and the univox superfuzz:  schem at geofex.com

[DDD]

Since we're discussing the CB configuration here, maybe it will be interesting to think of the common gate FET stage at the same time?

[A.S.P.]

sorry: but where is that common base in the univox?

[gaussmarkov]

sorry: but where is that common base in the univox?

i was referring to the differential pair smack in the middle of the schem.  is that OT?

[Sir H C]

Cascoding uses the common base.  I guess you could do a linear booster with the common base configuration, but you need a relatively high output impedance and relatively low input impedance to actually get the circuit to work.  Say 47 k on the emitter, base biased to about 1.7-2 volts, then about a meg on the collector.  Should give a gain of about 20, will probably be less.

[gaussmarkov]

sorry: but where is that common base in the univox?

i was referring to the differential pair smack in the middle of the schem.  is that OT?

sorry.  i inferred that differential pairs were necessarily common base.

Hey the common base is part of the differential pair, it is used a lot 

after reading and looking more carefully, i don't see that.

[Sir H C]

sorry: but where is that common base in the univox?

i was referring to the differential pair smack in the middle of the schem.  is that OT?

sorry.  i inferred that differential pairs were necessarily common base.

Hey the common base is part of the differential pair, it is used a lot 

after reading and looking more carefully, i don't see that.

Using Q1 and Q2 from this schematic:
http://gaussmarkov.net/layouts/bd2/schem.png
as the basis for the explanation:

So the fedback signal comes into Q2 at the gate.  This swings the source up and down in sync.  The source of Q2 then drives the source of Q1 with the fedback signal, helping linearize the discrete op-amp.  So Q1 is used as a common emitter relative to the input signal but it is common base relative to the fedback signal.

[gaussmarkov]

thanks very much!

[A.S.P.]

I`d still rather see a "to cathode" feedback into Q1...