Large Signal Transcondunctance Gm not gm

Discussion in 'General Electronics Chat' started by KCHARROIS, Apr 10, 2013.

Jun 29, 2012
292
1
Hello,

I've been reading up on bjt amplifiers and such and I understand the small signal transconductance gm but I can't find anything on large signal transconductance Gm which is not the same as gm. Is there another term that they use for Large Signal Gm because I can't find any info on it?

Thanks

2. t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
See attachment page 34 onwards ..

File size:
1.7 MB
Views:
230
3. tindel Active Member

Sep 16, 2012
576
196
Just a clarification in terminology so you don't get confused -

A BJT is not a transconductance amplifier (Iout is a function of Vin (Gm / gm) - it is a current amplifier (Iout is a function of Iin (HFE / hfe))

4. crutschow Expert

Mar 14, 2008
13,501
3,375
Not to start another tedious argument on the subject but the internal physics of a BJT is that it acts as a transconductance amplifier with a low input impedance between base and emitter.

The small-signal transconductance gain value (which varies inversely with emitter current) is typically used when analyzing the small signal characteristics of AC amplifiers.

For ease of calculating large signal, switching, and DC bias calculations, the current gain (Hfe or β) of the transistor is typically used since the BJT externally looks much like a current amplifier for large signals.

patricktoday likes this.

Jun 29, 2012
292
1
Ok well if gm = Ic/Vt, how does Gm = (Ic/Vt)*(1/Av)??

I can replace Ic/Vt with gm making Gm = gm/Av but again why does Gm = gm/Av?

...Gm and gm are different

6. crutschow Expert

Mar 14, 2008
13,501
3,375
Where did you get that information? I've never heard of two different gm's.

7. t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
A distinction is made between small signal and large signal transconductance. The study & analysis of oscillators may make use of the concept - particularly in the matter of oscillator output amplitude & frequency stabilization. It involves one delving into Bessel Functions which may not appeal to the average electronics person such as myself. The BJT Colpitts oscillator seems to be a classic candidate for this approach.

I refer you to the attachment in post #2. The same author has produced other documents which include another chapter VI on oscillators, which makes reference to chapter V equations involving the relationship between gm & Gm.

8. YokoTsuno Member

Jan 1, 2013
41
9
Honestly speaking I had never heard of Gm prior to this thread, only gm and Hfe/β of course.

When you search for this parameter you always end up in RF amplifier theory. The reason for the existance of Gm is, I assume, that since class C amplifiers are unbiased small signal transconductance gm (ΔIc/ΔVb) is a bit of a misnomer. It makes therefore sense to define a large transconductance Gm for such applications. t_n_k's pdf also defines it that context.

9. cabraham Active Member

Oct 29, 2011
82
30
The internal physics of a bjt describe the device as charge controlled. Charge distribution, depletion region, Fermi energy level, mobility of electrons and holes, transport, injection, etc. is what semiconductor physics involves. I have had 5 courses in semicon phy, 4 at the graduate level, and nowhere is it taught that a bjt is a "transconductance amp with low input Z". A bjt amplifies both I & V. It has a current gain hfe as well as a transconductance gm.

An active device is useful because it can provide current gain and voltage gain both greater than unity. Also, FWIW, "internal physics of bjt" defines the speed or bandwidth of the bjt as the transition frequency, ft, where the *current gain* has dropped to unity. A FET ft is defined the same way, current gain of unity.

Any FET or bjt amplifies both I & V. That is what makes them so useful. Take a single stage CE or CS amp. There are 4 bias resistors and 1 active gain device, FET or bjt. The stage exhibits current gain and voltage gain greater than unity. What is providing these gains > 1? It can only be the active device itself. Resistors do not provide gain.

These threads go along smoothly until some self-proclaimed expert hijacks it telling everyone that the bjt makers publish bad info, unis teach it wrong etc. The classic bjt operation described by OEMs and unis is spot on. I ask that this thread be allowed to continue w/o "experts" telling us how things "really work". Thanks.

Claude

Jun 29, 2012
292
1
Funny you mentioned oscillators specially colpitts, its this info that I need to understand them more.

Jan 2, 2016
2
0

12. aac30476 New Member

Jan 2, 2016
2
0
KCHARROIS,

Curious where this question came from. I ran into it in Ronald Quan's book on building transistor radios. He has an explanation that I currently cannot wrap my head around. The book was published after your post.

Are you still interested in this topic?

Thanks.