BJTs typically achieve the highest overall gain, especially in small-signal applications, (even more so with Darlington BJTs), due to their high current gain and transconductance. However, the actual gain depends on the specific circuit configuration and operating conditions. For precise comparisons, circuit parameters (e.g., load resistance, biasing) must be considered.
I meant clean, undistorted gain. Thank you for reminding me that when an engineer says "gain" it means the amplitude of the output signal divided by the amplitude of the input signal. When a guitarist says "gain" there is a range of definitions. Don't even get me started on "gainy."
So now that we've cleared that up, which transistor type do you think has the highest gain and which has the lowest?
I'd be tempted to adscribe to Cybercow's answer, however, I kinda feel there's a catch somewhere in the question. IME it is possible to get a JFet to exhibit the same gain as a BJT just by adjusting the components values around it, but the downside is loss of headroom. Haven't tinkered enough with enhancement Mosfets to have an answer about them.
No catch. I will present two solutions: one where all three transistors have the same collector/drain resistor value and run at the same collector/drain current. For the other solution, I tweak drain/collector resistors and currents get them to all match the gain of the highest gain device.
My intent with this discussion was to stir up interest and to attempt to answer questions like "why use a JFET? in this circuit?" or "how do I get the maximum gain?"
In some of my circuit mods, I replace a BJT gain stage with a JFET and I get asked "Why did you do that? What's the advantage?" Sometime the answer is straightforward because it serves a particular circuit requirement. Other times, I just do it for the Hell of it.
From my understanding, device gain will typically be either voltage or current - except MOSFETs which typically have a fairly high voltage gain and near if not infinite current gain; but have a lower transconductance than BJTs. BJTs tend to offer higher current gain. JFETs seem to offer moderate voltage gain but lower current gain than BJTs. JFET gain is limited by their lower transconductance. And all three device gains are dependent on the circuit configuration.
Since their gate current at audio frequencies approaches zero, it is meaningless to take about current gain with FETs. BJTs are at their core current mode devices. That doesn't me we can't measure or calculate voltage gain and transconductance in a BJT. Since the discussion has moved along so quickly, I'll reveal my simulation results in the morning. Unless you're on the other side of the International Date Line in which cast it's already tomorrow.
For equal drain or collector currents and equal collector or drain loads, BJT has the highest gain at 44dB, followed by MOSFET at 40dB and finally JFET at 32dB.
Here is a typical simulation. Not that there is no AC feedback and the only DC feedback is the emitter or source resistor bypassed by a large cap. These stages are running "wide open."
But what if we wanted to boost the gain of the JFET & MOSFET to match the BJT? Could we do that? Yes we can! The price we pay is higher drain load resistance. With BJTs, the transconductance is proportional to collector current, i.e. if we double the collector current then we double the transconductance. But unless we increase the rail voltage, we have to cut the collector load resistance in half to maintain headroom. So with BJT's it's a zero-sum game.
FETs are a different story. With FETs, the transconductance goes as the square root of drain current. That means if we double the drain current, we only increase the transconductance by 40%. But what if we cut the drain current in half? Then the transconductance goes down by 30%. Now we can double the drain load resistance, which makes the overall gain go up by 40%. That's about all we needed to get the MOSFET up close to 44dB. The JFET needs more of a gain boost, so I had to reduce the drain current about 15x and increase the drain load resistance by 15x.
CyberCow's point about load resistance is well taken. By increasing the JFET's load resistance by 15x, I reduced its ability to drive the next stage. If it's another JFET, then no problem. But if it's a tone-shaping network or another pedal, there might be difficulties.
This exercise shows that the practical limit for a BJT running on 9V is around 44dB. Beyond that, we have to either boost the rail or give up some headroom. also of note is the steeper bass roll-off of the BJT. That's due to the relatively low input impedance of a BJT. Here's where HFE can help. The input impedance of a BJT gain stage is proportional to HFE. 2N5089 are very high gain devices with typical HFE over 700. If I put in a 2N3904, we'd see more bass roll-off. MOSFETs have a very high (nearly infinite) input impedance, however they require biasing resistors, which lower the effective input impedance. It is not uncommon to see 10M bias resistors on a MOSFET.
I'm wrapping my head round this little guy. And I'm enjoying the pain. I'm ultimately not super well versed in EE, though. Some of the terminology goes a little over my head. Like...Transconductance. The ability for a device to convert voltage into current. New vocab word for stickman.
So...I'm trying to fit this into my framework of BJT vs FET. I realize I'm probably gonna lose some of the finer details here...but it's always useful to re-phrase concepts into my own words for this kind of stuff to stick in my brain.
BJT...current controlled device. Works kinda like a blade-less fan, where airflow along the perimeter of a big hole entrains air in the center of that hole. The area before the fan is the emitter, the airflow from the fan blades (because the name is a misnomer, it's got a squirrel cage blower in it) is the base current, and the total combined airflow from the fan blades + the entrained air exiting the fan assembly is collector current.
Not a perfect analogy. There's no delta P across the fan when air isn't flowing, and a typical BJT amplifier circuit will certainly have a delta V across the emitter and collector when current isn't flowing. Still, as a guide for an HVAC guy who bangs rocks, good enough.
FETS: keeping with the idea of using air as an analogy...I think of these as, like...a pneumatic actuator that is controlling a damper in a duct system.
The pneumatic actuator keeps the forces that control the actuator (pressure of compressed air) and the forces that the actuator is controlling (air pressure across a damper) separated.
In a direct-acting actuator, an increase in compressed air pressure will result in an increase in flow across the damper...but the two streams (compressed air and duct air) do not combine. Like ghost busters.
So...given that framework...we have two devices that have the ability to control current:
1) The first adds the current on the input that causes the change to the total output. (BJT). IC = IE + IB
2) Thr second isolates the current (and it's associated voltage) that causes the change from the output. IS=ID
Given the same power supply and circuit considerations to work with, it makes sense that BJTs would ultimately be better at producing more current gain.
Now...in terms of dB, I might be getting a little lost. I struggle a bit with that one. So...it's essentially a ratio between the power in, versus the power out, right? Which makes my brain hurt if thinking about how it's used in loudness measurements. Ow. But...it helps me to consider that were still dealing in terms of V*I.
In a FET, we can manipulate the dB gain by increasing the drain load resistance, which decreases the drain current but increases our voltage swing at Vds.
Simply because the output of a FET in +dB can be manipulated to the same +dB gain of the BJT, though, that does not mean that the FET can be used wherever a BJT is.
This is because we achieve the same gain only at the expense of current, which could potentially create a problem when driving another device with a low input impedance. Which, because we've got our drain squeezed off so much, will lead to an overall drop in voltage swing as the circuit struggles to supply the current that the next, lower impedance load demands.
The term dB refers to a ratio. It is dimensionless. Sometimes it refers to power, sometimes it refers to voltage, sometimes it refers to the intensity of a pressure wave in air with respect to a reference level. when I use the term dB in these forums, it is always a voltage ratio: usually Vout/Vin. To convert from a voltage ratio to dB, you take the base-10 log, then multiply by 20. Example: A gain of 50x is 34dB because log10(50) = 1.7 and 20 * 1.7 = 34. You have to have a calculator or access to a website that does dB conversion. dB is convenient in engineering because we can express very small or very large ratios with 2 or 3 digits. Also, the perception of loudness is logarithmic. Doubling the power does not make an amp sound twice as loud.
Transconductance is how much the drain (collector) current changes in response to a change in gate (base) voltage.
In JFETs, transconductance depends on drain current, Idss & Vp. Idss & Vp are determined by the manufacturing process.
In BJTs, transconductance depends only on collector current. It is the same for all part numbers, even the same for Si & Ge! It's a matter of physics.
Without consulting the oracle of the ethers, I'd have guessed BJT, 'cause you did say in the OP max headroom...
... so I was thinking "it depends on how the device is implemented in a circuit". Yeah. BJT. Further guessing I'd say JFET next, MOSFET after that... but unlike micmac, my suggested order is all based on a "feeling", accumulated info my brain's filtered from seeing these or those circuits over time and never thought about the specifics 'til this thread. So I got the MOSFET and JFET ackbwards...
@Stickman393 — pretty fancy rock-bangin', if you ask me. If you're bangin' rocks, then I must be...
Banging air, clapping my hands together without actually making the hands contact each other — or to use another analogy, shooting blanks.
