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?

 

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.

 

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?

 

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)

 

If you bias a Jfet at a certain current then its transconductance does not change when the temperature changes.