Getting the resistors right

[Nigel Goodwin]

It would be better to eliminate R3. I expect that the person who designed the circuit that was in the book was one of the "transistors are current controlled" school.

But in fact, they are voltage controlled.

It's an often argued point - but current 'controlled' makes a LOT more sense - and the transistor manufacturers even specify the current gain of their devices.

Asserting they are voltage 'controlled' makes very little sense, and just confuses the issue.

By simple ohms law it's blindingly obvious that you MUST have a voltage for there to be a current - so in that respect there is a voltage.

However, the fact remains, that a transistor base requires current, and the device gives current gain based on the current input to the base. Most of the world considers a transistor a current amplifier for this reason.

An FET on the other hand is a voltage 'controlled' device, you apply a voltage to the base, and the drain current varies accordingly - the gate draws no current, so just like a valve/tube it's a voltage driven device.

 

[ljcox]

Here is an extract from the 2N3904 data sheet.

See the graph entitled Base - Emitter On voltage versus Collector current.

This clearly shows that the collector current is controlled by the BE voltage.

It is an expotential function but as the graph has a log scale it is approx straight.

For example, on the 25 C curve, at Vbe = 0.6 Volt, the collector current is 0.1 mA.

At about 0.75 Volt, Ic = 20 mA.

If you study the fundamental operation of transistors, the collector current is controlled by the charge in the B-E region and since Q = CV, the charge is proportional to the BE voltage. So you can say it is voltage controlled.

This is why circuits such as the current mirror work.

There is no doubt that Ic = Beta Ib is very useful, but it does not mean that the Ic is current controlled. The base current is necessary to keep the BE charge constant since some charge carriers in the BE region are gobbled up by recombinations and become a component of the emitter current.

 

[UTMonkey]

Thanks for your notes Len, I will study these.