# Transconductance in Transistors

Discussion in 'General Electronics Chat' started by lkgan, Jan 16, 2010.

1. ### lkgan Thread Starter Member

Dec 18, 2009
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0
Hi everyone,

Why bipolar transistors have fewer nonlinearity problems compared to JFET? It's a fact that JFET has a lower transconductance than a BJT for the same current. How would the transconductance, gm affects the stability of a transistor? And how would the temperature variations affect the transconductance?

2. ### Audioguru New Member

Dec 20, 2007
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896
A transistor is almost always used with negative feedback to reduce its very high distortion. It has a high transconductance so reducing its gain with negative feedback still results in plenty of gain.

A FET also causes high distortion but its transconductance is low so not much negative feedback can be used to reduce its distortion. So its distortion will be higher than a transistor.

3. ### lkgan Thread Starter Member

Dec 18, 2009
117
0
Do you know what's the relationship between temperature variations and transconductance in a transistor? In what way can I know whether a BJT is more stable compared to JFET?

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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1,115
gm for BJT is equal:
gm=1/re without external emitter degeneration resistor (Re=0).
And with external emitter degeneration resistor gm=1/(re+Re)

And re=Vt/Ic when
Ic- collector current
Vt- thermal voltage
Vt=(KT)/q
K-Boltzmann constant k =1,380410^-23J/°K
q - elementary charge q =1,602110^-19C
T- absolute temperature in kelvin.
So Vt=(K/q)T=86.1619uVT
And for example now we can easy calculate Vt that for 25 ° C, ie T = 298.15 K
Vt=86.17uV298.15K=25.691mV but for practical calculation we assume Vt=26mV

And gm for JFET for Rs=0Ω

$gm=\frac{2*I_{DSS}}{|V_{gs(off)}|}*(1-\frac{Vgs}{V_{gs(off)}})=\frac{2*I_{DSS}}{|V_{gs(off)}|}*\sqr{\frac{Id}{I_{DSS}}$

And with external Rs resistor
gm_z = 1/( 1/gm+Rs )=gm/(1+gmRs)

Last edited: Jan 17, 2010
5. ### Audioguru New Member

Dec 20, 2007
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896
If you bias a Jfet at a certain current then its transconductance does not change when the temperature changes.