tube and transistor amp input impedance

[veenife]

hey, i ve been reading on the topic in some forums in the internet... i found some contradictions on what i read... so i want to clarify that...

so in one forum i read that tube amp input impedance is mostly a capacitive impedance that stays on the picofarad range (very small)....

in other forums ive seen that tube amp input impedance is a mix from the grid's resistor plus the tube capacitor's capacitance... and that will be mostly resistive impedance going some thing close to 1 megaohm ....

also i read that transistor amp have just resistive impedance that is much lower in comparison to tube amplifiers....( like so not frequency dependent) ...???

what is the truth of all that????

 

[Audioguru]

A tube amplifier can have an input impedance of a low resistance needing a huge input coupling capacitor value or it can be 1M ohms or more needing a low input coupling capacitor value. The input to ground capacitance is only about 10pF (mostly wiring capacitance) which is nothing to audio frequencies. The input impedance is the value of the grid resistor.

A transistor amplifier can have Jfets at the input if you want an input impedance of 1M ohms or more. Ordinary transistors can provide an input impedance of hundreds of ohms up to Megohms (like in most opamps).

You do not want to MATCH the input impedance of the amplifier to the output impedance of the source signal because that will load down the source producing distortion and it will throw away half of the signal level. It is good to have an input impedance of the amplifier 10 times or more higher than the source impedance.

 

[veenife]

so by that you mean that depending on the circuit build up from the amp a tube amp can have more capacitve impedance and in other cases more resistive impedance???

is "tube capacitance" related to the capacitor that smooths the alternation in voltage of the tube?

 

[LvW]

veenife, just for clarification and to avoid misunderstandings on your side:
In many cases it is the biasing network in front of the active unit (tube, FET, BJT) that determines the input impedance of the amplifier circuit.

 

[veenife]

yeah? ... good to know!

that already changes my perpective of the whole thing again...

i not even know what to ask anymore, i still cant grab the topic properly... is there any book that explains this whole amp thing better?

 

[LvW]

i not even know what to ask anymore, i still cant grab the topic properly... is there any book that explains this whole amp thing better?

Perhaps it would be wise at first to gain some knowledge how such amplifiers work and how (better: WHY) the must be biased at a certain DC operating point.
Then you will see which of the several contributors (bias resistors and/or input resistance of the active unit) will dominate and detrmine the overall input resistance.

 

[Audioguru]

The input capacitance of a vacuum tube or transistor is extremely small. It is about as much as the tiny capacitance in the wiring of a circuit. It has no effect on audio but it affects high frequency video or radio signals.

The input coupling capacitor of an amplifier blocks DC and low frequencies but passes higher frequencies. There is a simple formula to calculate the lowest frequency it passes without attenuation.
If the input resistance of an amplifier is high then the value of the input coupling capacitor can be low and still pass all audio frequencies. If the input resistance of the amplifier is fairly low then the value of the input coupling capacitor must be fairly high to pass all audio frequencies.

 

[veenife]

alright, that is definitely a plus on my understanding... tx!

so in this case the input resistance together with the coupling capacitor will work as a first order RC filter??

is the input resistance just equal in value as the grid resistor OR is the input resistance infact caused by the grid resistor itself???

 

[Audioguru]

A coupling capacitor feeding a resistor to ground (the grid resistor) is a first-order highpass filter. It blocks DC, attenuates low frequencies but passes higher frequencies.

The grid resistor's resistance is the input impedance of a tube amplifier.