Hi!
I would like to ask some questions:
1. Sometimes I see in voltage divider mosfet bias circuit a
capacitor connected between the middlepoint of the voltage divider to the ground. Sometimes not.
A, Is this cap really necessary or not??
B, Should be ELKO or foil cap?
C, Is there any rule for calculating such a capacitor's value?
2. Sometimes I see "Accelerator" capacitor on the amplifier PCB, in the input of V+ to ground. Sometimes not.
Sometimes it is a few hundred uF ELKO or a couple of nF fouil cap.
A, Is this cap really necessary or not??
B, Should be ELKO or foil cap?
C, Is there any rule for calculating such a capacitor's value?
3. This is an evergreen question 🙂
What kind of input cap is good?
In some cases I see ELKO in the input, but sometimes foil cap.
Which is better?
Greets:
Tyimo
I would like to ask some questions:
1. Sometimes I see in voltage divider mosfet bias circuit a
capacitor connected between the middlepoint of the voltage divider to the ground. Sometimes not.
A, Is this cap really necessary or not??
B, Should be ELKO or foil cap?
C, Is there any rule for calculating such a capacitor's value?
2. Sometimes I see "Accelerator" capacitor on the amplifier PCB, in the input of V+ to ground. Sometimes not.
Sometimes it is a few hundred uF ELKO or a couple of nF fouil cap.
A, Is this cap really necessary or not??
B, Should be ELKO or foil cap?
C, Is there any rule for calculating such a capacitor's value?
3. This is an evergreen question 🙂
What kind of input cap is good?
In some cases I see ELKO in the input, but sometimes foil cap.
Which is better?
Greets:
Tyimo
I will answer 3.
No cap is better than film is better than electrolytic
There is more to it than that but i don't want to ramble on
No cap is better than film is better than electrolytic
There is more to it than that but i don't want to ramble on
At least a third of Members are asleep at any time.
At least a third will be doing something else at any time.
Of the remainder, how many would you expect to be online when you post a question?
At least a third will be doing something else at any time.
Of the remainder, how many would you expect to be online when you post a question?
I wrote my reasking question after that I saw there was already more than 77 views....
But I am patient.🙂
If the voltage divider then feeds a large value resistor to the bias point (mosfet gate), then the answer is usually 'yes, a capacitor is sensible'.
Working with the voltage divider resistor, the capacitor forms a RC low pass filter to smooth the bias voltage. Use an electrolytic.
: )
RC filter.
If you are a member of ABFAB, then you can do without a capacitor, otherwise a capacitor is there to prevent amplification of dc, creating a large dc offset.
However if there is no dc at the input then you don't need a capacitor.
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Thanks Gordy! This is the first useful info!🙂
What do you mean on large value? 10K or higher?
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Melon Head!
The question was: What kind of input cap is good?
If we already need to use some......
Greets:
Tyimo
Yes, I know it. You are right, but your answer it is still a cliche. Sorry.🙂
The question was: What kind of input cap is good?
If we already need to use some......
Greets:
Tyimo
I already answered that.
Film is better than electrolytic any day of the week
Nelson used electrolytic because low input impedance (rc filter effect).
If the input impedance of the amp is low you might be forced to use electrolytic otherwise use a film cap.
Film is better than electrolytic any day of the week
Nelson used electrolytic because low input impedance (rc filter effect).
If the input impedance of the amp is low you might be forced to use electrolytic otherwise use a film cap.
Tyimo
Electronics 101
A capacitor let hi frequency pass trough it blocking the DC
The opposite is the inductor DC passes trough it blocking ac signals.
I think that in this case we are talking about capacitors as a way to reduce ripple and ac noise on the supply
As in all things you need to make compromises.
Electrolytic are cheap farad for farad if compared to other caps.
But they are "slow".
A paper and oil or bees wax capacitor may sound much better but cost is very hi.
You could use them as reservoirs capacitors but the cost will be maybe 100 to a 1000 time that of the electrolytic equivalent.
So you compromise and put a few microfarads of the better quality capacitors where needed.
You can use smaller electrolytic or foil or even tantalum.
They all do the job with same compromises.
(Tantalum are quite god as decoupling near the supply pins of IC they are much faster than the electrolytic equivalent still you will also see a smaller film capacitor next to it)
For example if you have a large load to carry for a long distance you will be better off with a diesel engine (lorry) if you need to save weight and go fast you may use a Wankel engine like in the RX7.
You forgot to mention the small 100 nano Farads across the bridge rectifier they work wonders in reducing noise. (Switch noise of diodes changing over).
Electronics 101
A capacitor let hi frequency pass trough it blocking the DC
The opposite is the inductor DC passes trough it blocking ac signals.
I think that in this case we are talking about capacitors as a way to reduce ripple and ac noise on the supply
As in all things you need to make compromises.
Electrolytic are cheap farad for farad if compared to other caps.
But they are "slow".
A paper and oil or bees wax capacitor may sound much better but cost is very hi.
You could use them as reservoirs capacitors but the cost will be maybe 100 to a 1000 time that of the electrolytic equivalent.
So you compromise and put a few microfarads of the better quality capacitors where needed.
You can use smaller electrolytic or foil or even tantalum.
They all do the job with same compromises.
(Tantalum are quite god as decoupling near the supply pins of IC they are much faster than the electrolytic equivalent still you will also see a smaller film capacitor next to it)
For example if you have a large load to carry for a long distance you will be better off with a diesel engine (lorry) if you need to save weight and go fast you may use a Wankel engine like in the RX7.
You forgot to mention the small 100 nano Farads across the bridge rectifier they work wonders in reducing noise. (Switch noise of diodes changing over).
O.K. You are right. But! Some people swear that a high quality ELKO like ELNA Silmic, Cerafine or Black Gate etc. are more dynamic and "thicker", "richer" sounding than a film cap.
Yes, I know the best is if I trying it by myself🙂
The two resistors of the voltage divider are effectively in parallel, and the 'new' capacitor shunts them to ground at AC signal level. Therefore the 'large value' resistor becomes the effective input impedance. As a FET gate takes almost no current the value of this 'large value' resistor can be high, typically 100k or more.
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