Dear Jan-Martin, LF group,
At 14:10 21/01/2003 +0100, you wrote:
A source follower has unity gain only with infinite load impedance. But
most source followers have a built-in load in the form of the source bias
resistor, which will normally be a few hundred ohms. A FET with
transconductance of a few mS has a source follower output impedance of a
few hundred ohms, so the bias resistor substantially loads the output,
resulting in gain well below unity. This can be avoided by increasing the
load impedance in different ways, eg. by connecting a high impedance choke
in series with the source resistor. In the compound follower circuit, the
low potential end of the 100R resistor is bootstrapped by the output of the
emitter follower, making its effective impedance much higher. The bipolar
follower has much higher transconductance, giving an output impedance of a
couple of ohms, and so can achieve nearly unity gain even with a 50 ohm
load. The actual gain of the complete circuit was about 0.9 - 0.95,
depending on the transistors used.
A source follower has incredible low gain - even when driving high impedance,
that's the reason I discard such constructions, but using a PNP instead of
you may achieve over unity gain. A BJT emitter follower has in the region of
0.8-0.95x voltage gain.
You set the feedback to adjust the gain, and add some postive feedback to
the input and achieve very high input impedance.
With a high impedance source, the thermal noise will be high also - eg. for
10k Rs, about 0.2uV in 300Hz. Most reasonable receivers have lower noise
floor than this, so increasing gain is not very useful. If you do need
higher gain, a circuit like LA8AK suggests is a good way to go.
The circuit I experimented with is not the best high impedance buffer money
can buy, but it is simple, requires no adjustment, and delivers nearly
unity gain with adequately low noise and low distortion for many purposes.
Cheers, Jim Moritz
73 de M0BMU