Claude Abraham
Member
1.) Common base or emitter, e- from the emitter are collected by collector. Emitter follower works the same way. The origin of Ic is the same for all 3 configurations.
2.) Why is it not convincing? FETs have current gain. But in a FET controlling Ig (gate current) is a poor way of controlling Id. A CCS (constant current source) across g-s of a FET results in a voltage ramp which will inevitably exceed the breakdown voltage of the g-s oxide. A FET cannot be current driven & must be voltage driven. I always regarded FETs as voltage driven because the device does not lend itself to current control/drive. Now regarding current gain of FET, it is different at every value of frequency. Unlike a bjt, the gate current in a FET, Ig, is all displacement current, not conduction current. The Ig is needed to charge/discharge g-s capacitance. At 2.0 kHz it requires twice the Ig value compared to 1.0 kHz.
Hence for FETs, we do not define a current gain, because it varies w/ frequency. Instead we define a transition frequency, f[sub]t[/sub], where current gain is unity. At lower frequencies, current gain is just f/ f[sub]t[/sub]. So a small signal FET may have a 300 MHz f[sub]t[/sub] value. When frequency is 300 MHz, current gain is 1, or Ig = Id. To implement a 1.0 mA current swing in the drain requires a full 1.0 mA current swing in the gate.
At a frequency of 3 MHz, the current gain is 100 (300/3). So for Id = 1.0 mA, Ig = 10 uA. Down at a mere 3.0 kHz frequency (human voice for example), the current gain is 100,000. If Id = 1.0 mA, Ig = 10 nA. Pretty straightforward if you ask me. FETs have current gain, but that does not make them "current controlled". Again, if we examine the semiconductor physics of a FET, it is charge controlled just like the bjt. FET is a majority carrier device, bjt is minority carrier device. Voltage control for FET, & current control for bjt work great until the speed approaches the internal time delays, or for switching mode operation. Then the charge control model is best for both devices.
I design many motor drivers, LED drivers, power supplies, etc., using FETs. Believe me a FET has a finite current gain. It requires gate current to switch states. To change Id we must change Vgs, but to do that we must first change Ig. Ig is necessary but it is not the parameter we directly control, Vgs is directly controlled, hence the VC label. Am I clear?
2.) Why is it not convincing? FETs have current gain. But in a FET controlling Ig (gate current) is a poor way of controlling Id. A CCS (constant current source) across g-s of a FET results in a voltage ramp which will inevitably exceed the breakdown voltage of the g-s oxide. A FET cannot be current driven & must be voltage driven. I always regarded FETs as voltage driven because the device does not lend itself to current control/drive. Now regarding current gain of FET, it is different at every value of frequency. Unlike a bjt, the gate current in a FET, Ig, is all displacement current, not conduction current. The Ig is needed to charge/discharge g-s capacitance. At 2.0 kHz it requires twice the Ig value compared to 1.0 kHz.
Hence for FETs, we do not define a current gain, because it varies w/ frequency. Instead we define a transition frequency, f[sub]t[/sub], where current gain is unity. At lower frequencies, current gain is just f/ f[sub]t[/sub]. So a small signal FET may have a 300 MHz f[sub]t[/sub] value. When frequency is 300 MHz, current gain is 1, or Ig = Id. To implement a 1.0 mA current swing in the drain requires a full 1.0 mA current swing in the gate.
At a frequency of 3 MHz, the current gain is 100 (300/3). So for Id = 1.0 mA, Ig = 10 uA. Down at a mere 3.0 kHz frequency (human voice for example), the current gain is 100,000. If Id = 1.0 mA, Ig = 10 nA. Pretty straightforward if you ask me. FETs have current gain, but that does not make them "current controlled". Again, if we examine the semiconductor physics of a FET, it is charge controlled just like the bjt. FET is a majority carrier device, bjt is minority carrier device. Voltage control for FET, & current control for bjt work great until the speed approaches the internal time delays, or for switching mode operation. Then the charge control model is best for both devices.
I design many motor drivers, LED drivers, power supplies, etc., using FETs. Believe me a FET has a finite current gain. It requires gate current to switch states. To change Id we must change Vgs, but to do that we must first change Ig. Ig is necessary but it is not the parameter we directly control, Vgs is directly controlled, hence the VC label. Am I clear?