Current output and FETs
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Posted by MRW on December 18, 2006, 2:51 pm
Please log in for more thread options Happy Holidays folks! It's good to relax for a couple of weeks, huh? I don't remember exactly what I read, but I recall a device that stated that its output is dependent on the output current, so essentially it was labeled current output device. Assuming if the current in the device is very small and requires amplification, do I avoid using FET opamps? I was just reading the differences between BJTs and FETs, and it says that FETs are voltage controlled, so the gate is essentially isolated from the source and drain. Thanks! | |||||||||||||||||||||||||
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Posted by Charles Schuler on December 18, 2006, 3:58 pm
Please log in for more thread options In JFETs, there is a gate-channel diode so you can get current. In MOSFETs, there is an SiO2 insulator between the gate and the channel so you will not get any gate current unless the gate is overvoltaged (that destroys the device). FETs are voltage controlled current sources and BJTs are current controlled. The choice of an OP AMP is often trivial, but it can be quite challenging ... entire books on this subject! | |||||||||||||||||||||||||
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Posted by PeteS on December 18, 2006, 4:12 pm
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Charles Schuler wrote: >> Happy Holidays folks! It's good to relax for a couple of weeks, huh?
>> >> I don't remember exactly what I read, but I recall a device that stated >> that its output is dependent on the output current, so essentially it >> was labeled current output device. Assuming if the current in the >> device is very small and requires amplification, do I avoid using FET >> opamps? >> >> I was just reading the differences between BJTs and FETs, and it says >> that FETs are voltage controlled, so the gate is essentially isolated >> from the source and drain. >
> In JFETs, there is a gate-channel diode so you can get current. In MOSFETs, > there is an SiO2 insulator between the gate and the channel so you will not > get any gate current unless the gate is overvoltaged (that destroys the > device). > > FETs are voltage controlled current sources and BJTs are current controlled. > > The choice of an OP AMP is often trivial, but it can be quite challenging > ... entire books on this subject! > > Well, according to the Ebers-Moll equation, BJTs are really voltage controlled (even though base current flows) ;) On the original topic; there is no *average* gate current; there can be _huge_ gate charging currents. Cheers PeteS | |||||||||||||||||||||||||
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Posted by on December 19, 2006, 10:29 am
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PeteS wrote: > Charles Schuler wrote:
> >> Happy Holidays folks! It's good to relax for a couple of weeks, huh?
> >> > >> I don't remember exactly what I read, but I recall a device that stated > >> that its output is dependent on the output current, so essentially it > >> was labeled current output device. Assuming if the current in the > >> device is very small and requires amplification, do I avoid using FET > >> opamps? > >> > >> I was just reading the differences between BJTs and FETs, and it says > >> that FETs are voltage controlled, so the gate is essentially isolated > >> from the source and drain. > >
> > In JFETs, there is a gate-channel diode so you can get current. In MOSFETs, > > there is an SiO2 insulator between the gate and the channel so you will not > > get any gate current unless the gate is overvoltaged (that destroys the > > device). > > > > FETs are voltage controlled current sources and BJTs are current controlled. > > > > The choice of an OP AMP is often trivial, but it can be quite challenging > > ... entire books on this subject! > > > > >
> Well, according to the Ebers-Moll equation, BJTs are really voltage > controlled (even though base current flows) ;) > > On the original topic; there is no *average* gate current; there can be > _huge_ gate charging currents. > > Cheers > > PeteS The Ebers-Moll equations say no such thing. The emitter current in a bjt may be expressed as: 1) Ie = (beta + 1) * Ib, or as 2) Ie = Ies * (e^(Vbe/Vt) - 1). Likewise the collector current may be expressed three ways. 3) Ic = alpha * Ie, or alternately, 4) Ie = beta * Ib, or we may combine equations 2) and 3) to obtain 5) Ic = alpha * Ies * (e^(Vbe/Vt) - 1). Equation 5) is the one being referred to above. It is derived from equations 2) and 3). Many critics when quoting Ebers-Moll, omit the "alpha" factor, which is all important. "Alpha" is the ratio of Ic to Ie, and measures how effective the bjt functions as a current amplifier, aka common base forward current gain. In a bjt, there are but 3 terminals, 2 of which constitute the control electrodes, namely base and emitter. Since current by its very nature is a *through* quantity, whereas voltage is an *across* quantity, there are 3 input quantities associated with the base and emitter terminals. They are base current Ib, base-emitter voltage Vbe, and emitter current Ie. Neither of these 3 quantitiea could exist without the other two, nor can either two exist without the other one. There are no exceptions, under static or dynamic conditions. Thus Ib, Vbe, and Ie are mutually and intimately related. The collector current may be expressed as a function of either of the three, Ib, Vbe, or Ie. To "control" the collector current, we must control one of the input variables, and let the others and the collector current be defined by the device characteristics and laws of physics. If we control Ib directly, then 3) Ic = beta * Ib, and 6) Vbe = Vt * ln((((beta + 1)*Ib)/Ies) + 1). The problem with this approach is known as "beta dependency", as beta varies widely with specimen and temperature. This approach is only used when the bjt is used as a switch, toggling betwen cutoff and saturated states. By driving the base with current greater than Ic/beta, using minimum worst case beta value, assures saturation. If we attempt to control Ic with Vbe, another problem exists. "Ies", the base-emitter junction reverse saturation current in the Ebers-Moll equation, is a very strong function of temperature. In fact, 7) Ies = Ieso * e^(a*(T-To)), where Ieso is the reverse saturation at a reference temperature (usually 25C or 298K, or 300K), and To is the ref temp. I've designed logarithmic amps using diodes and bjt's. For a 1N914 axial diode, Ieso is around 3 to 5 nanoamp, and "a", the temperature coefficient is about 0.35 neper / degree K. If a voltage source, let's say 0.65V dc, is impressed across the base-emitter junction, the current is given by Ie = Ies * (e^(Vbe/Vt) - 1), and of course, Ic = alpha * Ie. But Ies = Ieso * e^(a(T-To)). Observe as follows. The voltage Vbe is held fixed (voltage drive or control ), and Ic and Ie are established. The product of Vbe and Ie is *power*, which is dissipated by the bjt device. The temperature can only *increase* due to non-zero power. This temp increase results in an *increase* in Ies, the b-e junction reverse saturation current. Since Ie = Ieso * e^(a*(T-To)) * ((e^(Vbe/Vt)) - 1), Ie must increase due to the increase in Ies. The temperature will increase even further, so that Ies increases further, increasing temp further, etc. This is clearly a runaway condition. Any p-n junction, such as a diode, LED SCR gate to cathode, bjt base to emitter, cannot survive and must never be *voltage-driven*. Controlloing Ie and/or Ic with Vbe is futile, and results in thermal runaway. Under no circumstances are bjt's ever voltage-driven or controlled. If we attempt to control Ic by controlling Ie, the relation is given by 3) Ic = alpha * Ie. Alpha varies for around 0.98 to 0.998 over specimen and temperature range. Alpha is very stable and predictable. The base-emitter voltage is given by 6) Vbe = Vt * (ln((Ie/Ies) + 1)). The temperature behavior is markedly different. If we fix the emitter current Ie, Vbe is given above, and the power dissipated by the bjt is non-zero. As a result, the temperature must increase. An increase in temperature incurs an increase in Ies. But observing equation 6), an increase in Ies incurs a *decrease* in Vbe, the opposite of the voltage-controlled case. This is very desirable, because the power in the p-n junction will now decrease. Thermal stability is insured. To be fair, the base driven scenario above, is also thermally stable. The problem with base current control is beta-dependency, but it is thermally stable. The thermal characteristics of forward biased p-n junctions mandate the they are always "current-driven" or "current-controlled". This is what the phrase "current-controlled" means. We fix the *current* to some value, and the voltage is determined by physics and device characteristics. Both are equally important and Ic won't exist without both of them. Ditto for a FET. To summarize, it is impossible for collector current to exist in a bjt, without Ib, Vbe, and Ie, all together in unison. You can never have one without the other two. Neither of the 3 quantities Ib, Vbe, and/or Ie, is responsible for "causing" the other two, or Ic. The collector current may be expressed as a function of Ib, Vbe, or Ie. Just as Ic = beta * Ib, it is equally true that Ib = Ic / beta. Just as Ic = alpha * Ies * (e^(Vbe/Vt) - 1)), it is equally true that Vbe = Vt * ln((Ic/(alpha*Ies)) + 1). The order in which the variables appear does not imply a pecking order or "cause/effect" relation. Neither Ib, nor Vbe, nor Ie, can ever be the sole cause of Ic. All 3 work together. There is no pecking order. Does this help? Claude | |||||||||||||||||||||||||
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Posted by MRW on December 19, 2006, 11:54 am
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cabraham01@msn.com wrote: > The Ebers-Moll equations say no such thing. The emitter current in a
..<snip>.. > There is no pecking order. Does this help? > > Claude Thanks, Claude! That's a perspective that I haven't thought of before. | |||||||||||||||||||||||||
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> I don't remember exactly what I read, but I recall a device that stated
> that its output is dependent on the output current, so essentially it
> was labeled current output device. Assuming if the current in the
> device is very small and requires amplification, do I avoid using FET
> opamps?
>
> I was just reading the differences between BJTs and FETs, and it says
> that FETs are voltage controlled, so the gate is essentially isolated
> from the source and drain.