emitter follower oscillation
This question has been confusing me for a while.
I know that the emitter follower is to drive low impedence load.
But when I see some of my friends' VCO design (all use npn+L load, which should have low Rout), some of them use emitter follower to drive the next stage while others do not....
Does anyone know when should we use emitter follower, at least for VCO ?
Thank you very much...
For emitter follower output signal is always lower than input signal by 0.7V. If it is acceptable for your design you can use that.
You would use Emitter follower to drive
large capacitive loads, As you know the
the oscillation frequency of an LC oscillator
is 1/sqrt(LC), hence if you want to increase
oscillation frequency you have to decrease
L and C. For gigahertz range system, next stage load Cap can severely limit macimum achievable oscillation frequency. An advantage of using BJT transistor is that they are fast (ft is very high) so it is suitable for high frequency.
Another important issue
is that the tuning range of the VCO, if the constant
capacitive part of the LC tank (such as next stage
input load) can limit the tuning range of the VCO.
The drawback is that the input resistance of the BJT degrades the Q of the VCO which degrades in turn,
phase noise, power consumption, etc...
a) voltage gain is 1 (see!)
b) high current amplification
c) high input impedance (see!)
d) no phase change/inversion
makes sense now?
It is not always necessary true. 0.7 is only when is silicon.
Add an AMP is to block the VCO and the loading circuit. If the loading circuit drains too much current from VCO or capacitively loads the VCO, a AMP (or BUFFER) is always needed to be inserted between VCO and loading circuit.
If the loading circuit is uncertain, the AMP or BUFFER should have a good isolation ability to keep its input impedance fixed while the loading circuit's impedance is changing.
gggould,
You don't need to use emitter follower in circuits that work.
However, if you design new circuit and you don't know how next stage may interfeare with previous it is always good practice to separete stages by emitter follower and avoid possible problems.