Common grid tube amplifier

[htg]

Does it really work? The grid currents are so small - it is hard to believe that a common grid amplifier makes sense.

 

[FvM]

The gate currents of a common gate FET amplifier are even smaller.
It's "sense" doesn't depend on a low or high current of the common terminal.

 

[htg]

Do you have a true explanation how it works?

 

[FvM]

What do you mean with "true explanation"?
1. I know, that tube as well as FET and BJT common grid/gate/base amplifier actually work.

2. The operation of these circuits is ruled by simple equations with parameters, that can be extracted from the respective datasheets.

3. The basic property of these circuits is to have unity current gain and larger than unity voltage gain, in addition a low (1/gm) input impedance.

As a side remark: Unity current gain implies, that the (signal) current through the common terminal is low.

 

[keith1200rs]

I'm not sure a common grid amplifier makes much sense on its own but as the top valve (tube) of a cascode amplifier maybe?

Keith

 

[FvM]

The "sense" of an amplifier is to amplify a signal. Although the voltage/power gain is lower than of a BJT, a tube can amplify in
common grid circuit and it has been used, particularly for RF applications.

P.S.: E.g. triode E88C is intended for common grid amplifiers up to 1000 MHz.

 

[byteptr]

There are some amplifiers that must have low input impedance,high output impedance and operate at high frequencies, the common grid amplifier is the best choice. (Example: for loading high Q resonant circuit).

You can use common grid/gate/base amplifier up to ft of active component for making oscillators also.

There are many circuits that uses this configuration, so it works.

Best regards,

 

[htg]

Somehow no one has a good explanation how it works, so I think it is suspicious.

 

[keith1200rs]

Somehow no one has a good explanation how it works, so I think it is suspicious.

No need to be suspicious! It really just the same as common gate in a JFET or common base in a BJT. The different configurations are compared a bit here: **broken link removed** but being valves (tubes) there isn't so much written about them nowadays. I think low input impedance amplifiers are never going to be used as much as high input impedance ones. As part of a cascode though ...

Keith.

 

[FvM]

Somehow no one has a good explanation how it works, so I think it is suspicious.

Smells like ignorance.

Unfortunately you didn't yet manage to tell, which property of common grid amplifier (or the equivalent semiconductor circuits) needs explanation. In my opinion, the operation is crystal clear and can be easily understood from basic circuit theory.

 

[htg]

FvM, your answer smells like hypocrisy. If you understand how common grid amplifier works, clearly and concisely explain. But I do not see why it would work - a triode with grouded grid will behave like a diode. So it could only do something because of its nonlinear characteristic, but I do not think it could make a useful amplifier.

 

[keith1200rs]

It will work as an amplifier just as a common base or common gate amplifier would. By inputting a signal in to the cathode you will vary the grid/cathode voltage which will modulate the anode current.

Keith.

p.s. I cannot vouch for the accuracy of the simulation or the validity of the design - I just threw it together with a valve Spice model I found. I didn't actually "design" it.

 

[htg]

It is not the grid-cathode voltage that matters. If the potential of the grid is constant, then varying the cathode potential will slightly change the cathode-anode voltage, but I do not see why this setup would make a useful amplifier.

 

[keith1200rs]

It is not the grid-cathode voltage that matters. If the potential of the grid is constant, then varying the cathode potential will slightly change the cathode-anode voltage, but I do not see why this setup would make a useful amplifier.

It changes the anode current. This in turn changes the anode voltage. Depending on the anode load resistance it will be an amplifier.

Keith.

 

[htg]

What you say seems to confirm my suspicion that it is the non-linear characteristic of the anode current vs anode-cathode voltage that makes it an amplifier.
May be so.
By the way, what is dB/dB supposed to mean in the graph in your last link?

 

[keith1200rs]

What you say seems to confirm my suspicion that it is the non-linear characteristic of the anode current vs anode-cathode voltage that makes it an amplifier.

My formal education didn't include valves although I played with quite a few when I was young. My understanding is that a valve is a voltage controlled current source. So you should be thinking in terms of the grid/cathode voltage varying the anode current, not the voltage. The characteristics are non-linear towards cut off, but there is a reasonablly straight section as well.

http://www.vac-amps.com/tube_curve_EL84.htm
**broken link removed**

By the way, what is dB/dB supposed to mean in the graph in your last link?

It is simply that the fixed probe I added was called dB and the scale is in dB. If I had renamed the probe to be Vout it would have said Vout/dB.

Keith.

 

[htg]

I still find it strange. Clearly it is easier to change the potential of an ungrounded grid, than to change the potential of the cathode.
I still do not understand why grouded grid would be superior.

 

[FvM]

To describe the operation of a linear amplifier component, e.g. a vacuum triode, you don't need to refer to a particular circuit (common cathode, grid or anode). You can describe it with a simple equivalent circuit and analyze it's behaviour in different circuits.

The "first order" model is a voltage controlled current source. The voltage difference between grid and cathode is the controlling voltage. The current flows from anode to cathode. The linear model has only one parameter gm, called transconductance.

To refine the model, you may want to add an inner resistance Ro to the current source.

The real component is actually non-linear, this means that the linear model is only valid for a specific operation point, defined e.g. by an Ia and Vak value pair.

You can put the linear triode model into the different basic circuits and caculate it's behavior as an amplifier. For the common grid amplifier, you get a low input resistance r11 ≈ 1/gm, a current gain of 1 and a voltage gain of gm*(Ro || Ra), where Ra is the external anode load resistance.

I'm not sure however, if we walk on a common ground of electronic circuit theory (at least a very basic one).

I still find it strange. Clearly it is easier to change the potential of an ungrounded grid, than to change the potential of the cathode.
I still do not understand why grouded grid would be superior.

Don't confuse the question. You asked "does it really work?"! There wasn't a talk about "superior", just preference for special purposes.

 

[htg]

Well, maybe it works. But supposedly it is superior at high frequencies. I do not see why it would be so.

 

[FvM]

There are a least two answers:
- It's part of the said cascode circuit. Here it works as a "current buffer" with a low input impedance that increases the bandwith of the first stage.

- In contrast to a common cathode amplifier, the input electrode (cathode) is "shielded" from the output electrode (anode), resulting in a lower feedback capacitance.