20-02-2018, 12:19 PM
Hi Andy,
The reason I asked about this application is simple: you might be worrying about the performance of the current source for no good reason.
I'm guessing that this is being used to supply the tail current to a valve-based LTP at the front end of an audio amplifier?
I've no real experience of using valves as a LTP, so what follows comes from the perspective of transistor circuits. However, I don't think I'll be that wide of the mark.
Before we can design the right current source for a circuit, we need to know what purpose the current source is fulfilling. Note that "right" does not automatically equal "best" - over-engineering part of a circuit can be as much of a crime as under-engineering might be.
In a conventional amp, using a current source on the LTP brings 2 benefits:
1. Greatly improved common-mode rejection
2. Better rejection of noise on the negative PSU rail (assuming NPN or valves).
These come about because of this simple fact: if you vary the tail current, then the currents in the collector/anode load resistors vary - and of course, you're taking an output from at least one of these resistors.
Let's think about point 1 - CMRR - first of all.
This is important in a general-purpose op-amp, where the two input voltages could be placed anywhere within the range of allowable values (which is close to the entire +/- supply rails for many op-amps). With just a tail resistor, the tail current is obviously directly related to the common-mode input voltage so the common-mode rejection is terrible, but a current source fixes that.
But for an audio amp, there is very little common-mode signal. In fact, the common-mode signal is the audio input itself - perhaps only a volt or 2. It would be different if you were running the power amp at a lower overall gain - and indeed, a unity-gain buffer is the toughest challenge for an op-amp in terms of CMRR (and common-mode distortion, which FET op-amps often suffer from).
In short, the current source only needs to keep the current constant (hence, Zout high) over a very small range of voltages. With that in mind, you actually have to think pretty hard about including one in the first place. If your negative rail is in the order of -80V, then a 15k resistor might do the job as well as a current source would in the overall scheme of things. Actually, 5mA is a pretty high tail current for a transistor amp - 1 or 2 mA is more typical, so that's a 75k or 39k resistor.
How about point 2?
This is simple: if V- varies, then the voltage across the tail resistor also varies, which means the current in it must also be varying. And we already know what that means...
A current source obviously fixes this problem. How good does our current source need to be for this problem? Well, the noise on the -V rail will be 100Hz and harmonics thereof. Even the most basic current source will cope with low-frequency stuff like this. If you're really going for broke, you might look at ways to improve it further, but the sad truth is this: it's actually far easier to decouple the -V rail before it hits the current source. A resistor and capacitor is far cheaper and dependable in mass-production than complicated current sources.
In fact, let's say that you didn't want a current source at all. Above, I said that a simple tail resistor might be adequate when the supply rails are high (high rails means large tail resistors, and large resistors connected to large supply rails look awfully like current sources in my book). If you did this but found the PSU rejection was poor on the -ve rail, a 1k resistor and a big cap ahead of the tail resistor would fix it - cheaply and elegantly.
The right way to do this is to first establish what sort of performance you need from your current source. So I'd build up the whole amplifier, but using just a tail resistor, and then inject a signal into the tail at different levels and frequencies, and see what sort of levels causes that source to appear at the output above the noise floor. Bear in mind you've got negative feedback on your side! Having done that, you now have a spec for the current source, and you can target your efforts into achieving this. As there's every chance that a simple "ring or two" would be well in excess of the actual requirements, that saves getting stuck in the rabbit hole.
Looking at the articles in post #13, the ring-of-two does pretty well. But given the high voltage/valve nature of this amplifier, I think I'd be tempted to use a MOS-FET as the second transistor
The reason I asked about this application is simple: you might be worrying about the performance of the current source for no good reason.
I'm guessing that this is being used to supply the tail current to a valve-based LTP at the front end of an audio amplifier?
I've no real experience of using valves as a LTP, so what follows comes from the perspective of transistor circuits. However, I don't think I'll be that wide of the mark.
Before we can design the right current source for a circuit, we need to know what purpose the current source is fulfilling. Note that "right" does not automatically equal "best" - over-engineering part of a circuit can be as much of a crime as under-engineering might be.
In a conventional amp, using a current source on the LTP brings 2 benefits:
1. Greatly improved common-mode rejection
2. Better rejection of noise on the negative PSU rail (assuming NPN or valves).
These come about because of this simple fact: if you vary the tail current, then the currents in the collector/anode load resistors vary - and of course, you're taking an output from at least one of these resistors.
Let's think about point 1 - CMRR - first of all.
This is important in a general-purpose op-amp, where the two input voltages could be placed anywhere within the range of allowable values (which is close to the entire +/- supply rails for many op-amps). With just a tail resistor, the tail current is obviously directly related to the common-mode input voltage so the common-mode rejection is terrible, but a current source fixes that.
But for an audio amp, there is very little common-mode signal. In fact, the common-mode signal is the audio input itself - perhaps only a volt or 2. It would be different if you were running the power amp at a lower overall gain - and indeed, a unity-gain buffer is the toughest challenge for an op-amp in terms of CMRR (and common-mode distortion, which FET op-amps often suffer from).
In short, the current source only needs to keep the current constant (hence, Zout high) over a very small range of voltages. With that in mind, you actually have to think pretty hard about including one in the first place. If your negative rail is in the order of -80V, then a 15k resistor might do the job as well as a current source would in the overall scheme of things. Actually, 5mA is a pretty high tail current for a transistor amp - 1 or 2 mA is more typical, so that's a 75k or 39k resistor.
How about point 2?
This is simple: if V- varies, then the voltage across the tail resistor also varies, which means the current in it must also be varying. And we already know what that means...
A current source obviously fixes this problem. How good does our current source need to be for this problem? Well, the noise on the -V rail will be 100Hz and harmonics thereof. Even the most basic current source will cope with low-frequency stuff like this. If you're really going for broke, you might look at ways to improve it further, but the sad truth is this: it's actually far easier to decouple the -V rail before it hits the current source. A resistor and capacitor is far cheaper and dependable in mass-production than complicated current sources.
In fact, let's say that you didn't want a current source at all. Above, I said that a simple tail resistor might be adequate when the supply rails are high (high rails means large tail resistors, and large resistors connected to large supply rails look awfully like current sources in my book). If you did this but found the PSU rejection was poor on the -ve rail, a 1k resistor and a big cap ahead of the tail resistor would fix it - cheaply and elegantly.
The right way to do this is to first establish what sort of performance you need from your current source. So I'd build up the whole amplifier, but using just a tail resistor, and then inject a signal into the tail at different levels and frequencies, and see what sort of levels causes that source to appear at the output above the noise floor. Bear in mind you've got negative feedback on your side! Having done that, you now have a spec for the current source, and you can target your efforts into achieving this. As there's every chance that a simple "ring or two" would be well in excess of the actual requirements, that saves getting stuck in the rabbit hole.
Looking at the articles in post #13, the ring-of-two does pretty well. But given the high voltage/valve nature of this amplifier, I think I'd be tempted to use a MOS-FET as the second transistor
