MJE340 and MJE350 in VAS stage

[rtarbell]

I've seen these two transistors from ONSemi used in the VAS stage of several high power amps. They've got the voltage and current carrying capability, and a very good power rating.

Their one main drawback is their Early voltage (the spice model for the MJE340 lists the Early voltage as something like 27 volts). This would mean a very low output impedance (and thus a very low gain for a VAS stage).

==> Am I looking to deeply into their flaw? Is the Early voltage not that important in a power amp for the VAS stage?

Thank you!
-RT

[AndrewT]

Hi,
Onsemi do not quote speed fT nor capacitance Cob.
Are they too embarassed to divulge?

As a VAS most seem to recommend low Cob. There are many 5W to 20W devices that have 1.7pF to 5pF output capacitance and also fT 100Mhz and way above.

You either arrange your own testing, or hope, or buy something else with a fuller specification.

BTW. what is early voltage and how is it specified?

[jacco vermeulen]

Early voltage = I * dV/dI in the I-V graph for the upper point on the linear section of the I-V curve.

[jcx]

your mistake is giving weight to spice model parameters, motorola isn't particularly bad - its just the state of free manufacturer's model accuracy is generally abysmal

try to sim the datasheet curves to get a feel for the (lack) of model accuracy

it is likely that the load driven by the vas dominates the nonlinearity

you might search for aksa's vas sugestions, sanyo and toshiba as I recall

[AndrewT]

Hi,
I've looked at the Vce-Ic curves and at Ic/Ib=5 top end I find 0.45A to 0.5A for Vce of 0.8V to 0.9V. The slope over this short section dV/dI = 0.1/0.05 = 2 and then using Jacco's formula I*dV/dI~=1 Where does Early voltage=27 come from? for the Ic/Ib=10 comparable figures are 0.7V to 1V for 0.2A to0.28A giving IdV/dI=0.9.

What am I misunderstanding.

No, I have not tried to interpret the saturated curves nor the Spice model listing.