Re: FET basics
On 25 Jan, 11:43, vic <n...@xxxxxxxxxxxxx> wrote:
John Popelish wrote:
Second, various online lessons I've found about FETs say that Ids is a
function of Vgs. Here Ids is fixed by TR3, so what is varying ?
In this case, the average current is regulated by TR3, while variations
around that average are controlled by TR1.OK, so the variations of Vs are applied on capacitor C5, which causes a
current proportional to it's capacity, and this current in turn causes
the voltage across R1 to vary. Is this correct ?
It helps to think about AC (signal) and DC (average / bias) conditions
seperately. Capacitors are effectively open-circuit to DC, and
low-impedance to AC. Think of the signal as a small AC component
superimposed on top of the steady-state DC average.
If it were not for C5, Vgs and Id would be constant, and the input
signal would appear on Vs.
The amount of charge stored on C5 is related to the voltage across it
by:
Q = CV
To change this voltage, you have to move charge. Current is the rate
of change of charge:
I = dQ/dt
You need a big current for a short time, or a small current for a long
time. At signal freqeuncies, we do not have a long time, and the
voltage cannot change fast enough in response to cycles of input
freqeuncy, so C5 keeps Vs fairly constant. This means the AC input
signal appears across Vgs, and this forces Id to vary in sympathy.
.
Relevant Pages
- Re: Standing-Wave Current vs Traveling-Wave Current
... If both the capacitor and the inductance are ... at a node to which a capacitor is connected. ... half of the cycle, both are removing charge. ... The capacitor voltage goes ...
(rec.radio.amateur.antenna) - Re: Standing-Wave Current vs Traveling-Wave Current
... Although the argument about no energy crossing the zero-current node is compelling, I don't feel that an adequate argument has been given to justify the wave "bouncing" theory over all other possible explanations. ... A consequence of this is that either we have a whole inductor with zero current, or the zero current point occurs between inductors, at a node to which a capacitor is connected. ... So for half of the cycle, both are putting positive charge in the capacitor, and for the other half of the cycle, both are removing charge. ... The capacitor voltage goes up and down as a result, as we can also see by looking at the voltage at this zero-current point. ...
(rec.radio.amateur.antenna) - Re: Capacitor charging configurations
... When connected together C1 will charge C2, and the steady-state voltage of each will be 0.5 volts. ... and in lamens terms we can describe it by saying the charge in C1, which produces 1 volt there, will distribute itself evenly among the total capacitance and result in half of the charge left in C1 and half in C2, thus producing 0.5 volts in each capacitor. ...
(sci.electronics.design) - Re: Capacitor charging configurations
... When connected together C1 will charge C2, and the steady-state voltage of each will be 0.5 volts. ... and in lamens terms we can describe it by saying the charge in C1, which produces 1 volt there, will distribute itself evenly among the total capacitance and result in half of the charge left in C1 and half in C2, thus producing 0.5 volts in each capacitor. ...
(sci.electronics.design) - Re: Cpacitor discharge
... Take a cap, charge it, disconnect it and let is sit there static. ... He wants to know how long it will take for the cap to reach 2.4 Volts. ... the discharge function of the capacitor must be a function ... of the instantaneous voltage on the capacitor. ...
(sci.electronics.design) |
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