Author Topic: Three stage amplifier  (Read 2013 times)

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Offline promach

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Re: Three stage amplifier
« Reply #25 on: March 09, 2018, 02:32:22 pm »
Is it true that reducing feedback factor or gain-bandwidth product could reduce power consumption ?
 

Offline amspire

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Re: Three stage amplifier
« Reply #26 on: March 09, 2018, 02:55:02 pm »
Is it true that reducing feedback factor or gain-bandwidth product could reduce power consumption ?
Gain-bandwidth product of an amplifier by itself - yes, you can lower power consumption and it will probably lower the gain bandwidth.

Don't see what feedback factor has to do with lowering power consumption. The lower the feedback factor, the higher the gain, so for a given input, the higher the output voltage amplitude. That would increase consumption a little.

Slowing down an amplifier can mean you can have much higher DC gain in the amplifier if you want it stable at all gains. If you take an amplifier that has a pole at 10 Hz and another at 1MHz, you probably want to have the 0dB point below 1MHz so you keep away from the chance of instability, so that means you do not want the amplifier gain to be more then 100,000 (100dB). If you can move the first pole to 0.01Hz while leaving the second pole at 1MHz, you can now have a gain of 100,000,000 (160dB) and be stable. This is great for precision circuits where you do not need speed. The cheapest way to reduce that first pole frequency is to lower currents in the first stage. This probably also reduces input currents. The other way to slow the amplifier down is to make the miller capacitance in the second stage 1000 times bigger and that is very expensive in an IC design.
« Last Edit: March 10, 2018, 12:25:15 am by amspire »
 

Offline Wimberleytech

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Re: Three stage amplifier
« Reply #27 on: March 09, 2018, 10:12:20 pm »
Is it true that reducing feedback factor or gain-bandwidth product could reduce power consumption ?

Reducing GBW will reduce power consumption.

For a miller-compensated (single miller) CMOS amplifier, GBW = gm1/C  (where gm1 is the transconductance of the first stage and C is the miller cap), and gm is proportional to sqrt(I).

Therefore, reducing the gm by a factor of 2 (e.g.), reduces the current by a factor of 4 (to a first order).
« Last Edit: March 09, 2018, 10:18:10 pm by Wimberleytech »
 
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Offline promach

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Re: Three stage amplifier
« Reply #28 on: March 10, 2018, 12:17:56 am »
Quote
Yes, playing with the input stage current will affect GBW, as well as a bunch of other performance figures.

However, adjusting GBW is typically done by resizing the Miller capacitance, which does not change power consumption. Therefore, your statement:

    Reducing GBW will reduce power consumption.

Is not necessarily true. Instead flip it around: reducing power consumption will reduce GBW (unless the Miller capacitance is adjusted to match).

@Wimberleytech

Someone commented to me about the above. What do you think ?
 

Offline Wimberleytech

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Re: Three stage amplifier
« Reply #29 on: March 10, 2018, 07:56:27 am »
Quote
Yes, playing with the input stage current will affect GBW, as well as a bunch of other performance figures.

However, adjusting GBW is typically done by resizing the Miller capacitance, which does not change power consumption. Therefore, your statement:

    Reducing GBW will reduce power consumption.

Is not necessarily true. Instead flip it around: reducing power consumption will reduce GBW (unless the Miller capacitance is adjusted to match).

@Wimberleytech

Someone commented to me about the above. What do you think ?

Yes, they are correct, increasing the miller capacitance will reduce GBW with no change in power.  Note, however, that I qualified my comment with the explanation about the relationship between gm, current, and GBW.  GBW, power, miller capacitance, slew rate, input-referred noise...all interrelated.

I can recommend this book: https://www.amazon.com/Analog-Circuit-Electrical-Computer-Engineering/dp/0199765073
Read the chapter on Operational amplifier design.  It does a great job of explaining the interrelationships.
« Last Edit: March 10, 2018, 08:03:15 am by Wimberleytech »
 
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Offline promach

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Re: Three stage amplifier
« Reply #30 on: March 10, 2018, 03:28:21 pm »
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The absolute value of this transconductance equals half the transconductance of each side of the input pair.

the input impedance rp3//cp3 equals half that of the individual transistors T2a and T2b

For this simple two-stage amplifier circuit , could anyone help to explain the two sentences above ?

 

Offline Wimberleytech

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Re: Three stage amplifier
« Reply #31 on: March 11, 2018, 01:39:28 am »
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The absolute value of this transconductance equals half the transconductance of each side of the input pair.

the input impedance rp3//cp3 equals half that of the individual transistors T2a and T2b

For this simple two-stage amplifier circuit , could anyone help to explain the two sentences above ?


See attached analysis.  The 1/2 comes from the fact that half of the input voltage w/r to ground appears across the control terminals (e.g., Vgs).
The attached analysis is done using large-signal notation and method--hoping this will be easiest for you to grasp.
 
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Offline promach

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Re: Three stage amplifier
« Reply #32 on: March 11, 2018, 01:57:54 am »
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the input impedance rp3//cp3 equals half that of the individual transistors T2a and T2b

Thanks for the transconductance explanation, but what about the input impedance ?
 

Offline Wimberleytech

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Re: Three stage amplifier
« Reply #33 on: March 11, 2018, 04:29:07 am »
Quote
the input impedance rp3//cp3 equals half that of the individual transistors T2a and T2b

Thanks for the transconductance explanation, but what about the input impedance ?

Well, I think it is wrong.  Should say "admittance."

What is this reference?
 

Offline promach

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Re: Three stage amplifier
« Reply #34 on: March 11, 2018, 11:42:37 am »
Quote
Should say "admittance."

@Wimberleytech
Could you elaborate ?

The book is page 61 of "Frequency Compensation Techniques tor Low-Power Operational Amplifiers"
 


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