🚀 go-pugleaf

Duncan Barclay wrote:
> 
> This thread started with someone (Nightjar) asking how they worked.
> Out of four replies I followed up on one thread was not asking how

Getting back to NightJar ... 

BJTs can be thought of a CURRENT CONTROLLING DEVICES 
and valves can be thought of VOLTAGE CONTROLLING DEVICES. 

BJTs offer variations in the emitter-base CURRENT controlling 
variations in emitter-collector CURRENT.  One important parameter 
for considering how BJTs operate is called the Beta, or Hfe which 
is equal to the ratio of a change in the collector CURRENT to a 
change in the base CURRENT while the VOLTAGE between the collector 
and emitter is held constant.  An important chart that you should 
be familiar with is the BJT Characteristic Curve chart. It associates 
differing base CURRENTS to collector CURRENTS. Actually, considering 
BJTs as CURRENT CONTROLLING DEVICES is really one of the only ways 
when one has to design for rad-hard (radiation hardened) situations. 

For VALVES, there is a parameter called the AMPLIFICATION FACTOR which 
is equal to the ratio of a change in PLATE VOLTAGE to a change in 
GRID VOLTAGE.  You should become familiar with the Characteristic 
Curves for Valves.  It's based on varying different Plate VOLTAGES 
and Grid VOLTAGES producing different Plate CURRENT. 

I've seen situations where FETs can be used directly in place of valves. 
In fact, plugged right into the socket of the tube.  But, that's due to 
the fact that FETs operate similarly to valves.  For example, a
parameter 
used for FETs is called the Amplification Factor and is simply the ratio 
of a change in Drain to Source VOLTAGE to a change in Gate to Source 
VOLTAGE (imagine that). 

You should become familiar with the Characteristic Curves for FETs. 
It's based on varying different Drain to Source and Gate VOLTAGES 
producing varying Drain CURRENTS in a similar fashion to Valves. 

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