>so it will be clearer
That's fine. So where IPS ends and VAS starts? What criteria determines the boundaries?
What I can see in the literature, there isn't strict rules by which individual stages are identified. More commonly, components are grouped in stages in a way that makes their operation more understandable. Nothing bad in adhering to definitions but the clarity of circuit description is the primary goal.
That's fine. So where IPS ends and VAS starts? What criteria determines the boundaries?
What I can see in the literature, there isn't strict rules by which individual stages are identified. More commonly, components are grouped in stages in a way that makes their operation more understandable. Nothing bad in adhering to definitions but the clarity of circuit description is the primary goal.
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Who's against it ? It is only necessary to avoid the negative consequences of using such a cascade.
Hi Stanislav, thanks for sharing the whole circuit. Now I can comment.
To me this looks like a single gain stage amplifier.
VT1,VT2,Q15, Q16 and R45 (which was missing in your original picture) is a GM cell with GM ~1/R45 = 1/20.
VT5 is a folded cascode that takes the output current of the GM cell (collector of Q16) and conveys it to the HIZ 'node', the collector of VT5.
So, this circuit has no second gain stage or VAS as many here call it.
It is not an axiom.
Only true is you drive it with a voltage source with low impedance. In this case, the gain is GM*RL.
If driven with a current source, like the output of a GM cell, it is a current buffer with current gain of alpha = beta / (beta + 1).
The input stage ends at the first HiZ node. So if you want to know how many gain stages an amp has, just count the number of HiZ nodes.
100%
The gain of this amplifier is as follows:
~ GM_INPUT_STAGE X GAIN_LOSS AT FOLDED CASCODE X R_HIZ
~ 2 x 1/(re_319 + 18) x 1/2 x 680 / (680 + re_315) x beta_315/(1+beta_315) x R_HIZ
The first factor of 2 is due to the complementary IPS. The 1/(re_319+18) is the effective GM of the diff-pair where 18 is the value of R319. 1/2 is because only one side of the diff-pairs is used. 680 / (680 + re_315) is the output current loss of the diff-pair due to small signal current getting shunted away from Q315's emitter by R327. R_HIZ is an abstraction of the impedance at the collector of Q315.
Now, simplifying a bit, Beta/(1+beta) = ~1, 680 / (680 + re_315) = ~1 if this thing is properly designed.
Thus the gain simplifies to:
G = 1/(re_319 + 18) * R_HiZ.
As you can see, Q315 offers no voltage amplification. It is the GM of the IPS x the impedance of the HiZ node.
I hope this is clear.
Regarding the SQUARE LAW amp, there is an LT1056. This is a 2 gain stage op-amp, so the so-called VAS in inside the op-amp.
BTW, can we stop using the term 'VAS'? This is the ridiculuous term that Self coined. All non-audio literature uses the name 'second gain stage'.
The gain of this amplifier is as follows:
~ GM_INPUT_STAGE X GAIN_LOSS AT FOLDED CASCODE X R_HIZ
~ 2 x 1/(re_319 + 18) x 1/2 x 680 / (680 + re_315) x beta_315/(1+beta_315) x R_HIZ
The first factor of 2 is due to the complementary IPS. The 1/(re_319+18) is the effective GM of the diff-pair where 18 is the value of R319. 1/2 is because only one side of the diff-pairs is used. 680 / (680 + re_315) is the output current loss of the diff-pair due to small signal current getting shunted away from Q315's emitter by R327. R_HIZ is an abstraction of the impedance at the collector of Q315.
Now, simplifying a bit, Beta/(1+beta) = ~1, 680 / (680 + re_315) = ~1 if this thing is properly designed.
Thus the gain simplifies to:
G = 1/(re_319 + 18) * R_HiZ.
As you can see, Q315 offers no voltage amplification. It is the GM of the IPS x the impedance of the HiZ node.
I hope this is clear.
Regarding the SQUARE LAW amp, there is an LT1056. This is a 2 gain stage op-amp, so the so-called VAS in inside the op-amp.
BTW, can we stop using the term 'VAS'? This is the ridiculuous term that Self coined. All non-audio literature uses the name 'second gain stage'.
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Это аксиома.
Axiom .
The advantages of the circuit are stable temperature and frequency properties, that is, the circuit parameters (voltage gain, current and input resistance) change slightly when the ambient temperature changes, and the output differential resistance is high.
The disadvantages of the circuit are the low input differential resistance and the lack of current gain, since a<1
Hi, Sandro!
Unfortunately this is a goalness.
Seeing amp as a electric principal there are a couple of different approaches to organize IPS-VAS-OPS orchestration.
But seeing amp as a mathematical gain block followed by current booster are nonsensitive for gain block name, so let it be commonly accepted VAS.
Exactly right, I still remember the crazy/pointless discussions about "Is it a TIS or is it a VAS"?
Why pointless? Because both camps were actually correct.
Anyway, slap me if you see me use the term VAS.
In absence of clear definition it is pointless to argue what belongs to VAS. Yet, VAS is in the title of this thread and hints (although not very precisely) certain properties of an amplifier stage discussed.
To my understanding, VAS roughly refers the first stage which provides full voltage swing on its output.
I like the way @sandrohv stressed the importance of identifying Hi Z nodes.
To my understanding, VAS roughly refers the first stage which provides full voltage swing on its output.
I like the way @sandrohv stressed the importance of identifying Hi Z nodes.
Let's split hair little bit, purely academic. It's still valid to say VT5 is voltage amplifier. Current from GM cell is converted to voltage by the (low) input impedance of VT5 (emitter). This small voltage is then amplified by VT5.
The input stage of a feedback amplifier has a non-inverting input and an inverting input. It converts the voltage difference between these two inputs to a current which is sometimes wisely called the error current.
The input stage stops at the point where this current is delivered. This is here that the next stage begins. The error current can be exploited using different ways,
Could an input stage work differently ?
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