Where Does Amplification Occur in Vacuum Tubes? A Detailed Analysis

In summary, an electron is emitted from the hot cathode via thermionic emission (Richardson's equation, Dushmanns law), and a large positive potential on the plate will draw all the emission electrons to the plate. The grid itself does not itself intercept the electrons, but creates a space charge density (very slow electrons) that limits the plate current (Child's law). Modulating the grid voltage modulates the plate current.
  • #1
vikram789
1
0
hi, we say amplification µ=plate resis.(rp) x gm = δVp/δVg=plate voltage/grid voltage
but I am confused where is the amplification taking place it is just that to maintain the same plate current we have to EXTERNALLY increase the plate voltage,δVp, in response to an decrease in grid voltage,δVg. Also i have one more doubt that, since we do not make the grid voltage positive w.r.t. cathode so amplification cannot take place as the presence of -ve voltage on grid will only restrict the no. of electrons reaching the plate.
 
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  • #2
Triode tubes amplify a signal placed on the grid that is biased above the cathode. (so you are correct that -v on the grid will restrict electron flow) The plate potential should be above the grid bias. Hence you vary the plate current by varying the grid potential.

Welcome to PF
 
  • #3
Given amplification µ=plate resis.(rp) x gm = δVp/δVg,
You say that in order to maintain a constant plate current, a decrease in δVg should be responded with an externally driven increase in δVp.

In order to maintain [tex]\mu[/tex] (amplification) one would have to maintain the ratio δVp/δVg. I think the physical idea is that to maintain the current for a given initial δVp/δVg, the difference in voltage at the plate and grid must stay the same. An increase in one would result in an increase in the other, while affecting other physical properties of the circuit.

As for the cathode and grid relationship, I'm not 100% sure, but I think a decrease in δVg increase flow from plate to grid, but restricts flow from the grid to the cathode. If δVg < δVc, then is the "cathode" even functioning as a cathode anymore?

Also,
If anyone wants to talk about the physics of music recording electronics, I would have fun with that.

-Nate
 
  • #4
Notice this thread is over 3 years old.

Here's a cute little tube animation:

http://www.ieeeghn.com/wiki/images/2/2b/Vacuum_tube.swf" [Broken]

A better site for tube info:

http://en.wikipedia.org/wiki/Vacuum_tube" [Broken]
 
Last edited by a moderator:
  • #5
Thanks! The video is cool.
 
  • #6
here is a review of vacuum tube operation, including some real equations:

http://www.john-a-harper.com/tubes201/

http://en.wikipedia.org/wiki/Space_charge

Electrons are emitted from the hot cathode via thermionic emission (Richardson's equation, Dushmanns law)

A large positive potential on the plate will draw all the emission electrons to the plate. The grid itself does not itself intercept the electrons, but creates a space charge density (very slow electrons) that limits the plate current (Child's law). Modulating the grid voltage modulates the plate current.

The plate voltage signal is produced by the plate current flowing through (across) the external plate circuit resistance (impedance). This can be a resistor or an LC circuit, or an output transformer primary.

Bob S
 
  • #7
Bob S said:
here is a review of vacuum tube operation, including some real equations:

http://www.john-a-harper.com/tubes201/

http://en.wikipedia.org/wiki/Space_charge

Nice. :cool:

I remember going through the tube base diagrams many times in this old book. I bet you have too. :smile:

47handbook.jpg
 

1. What is amplification in vacuum tubes?

Amplification in vacuum tubes is the process of increasing the strength of an electrical signal using vacuum tubes. This is achieved by controlling the flow of electrons through the vacuum tubes, which results in a larger output signal compared to the input signal.

2. How do vacuum tubes amplify signals?

Vacuum tubes amplify signals through a process called electron amplification. This involves using a heated cathode to release electrons, which are then accelerated towards an anode by a high voltage. The anode is designed to attract and collect the electrons, resulting in a larger output signal.

3. What are the advantages of using vacuum tubes for amplification?

Vacuum tubes offer several advantages for amplification, including high power handling capabilities, low noise levels, and a wide frequency range. They also have a natural compression effect, which can be desirable for certain types of audio amplification.

4. What are the common types of vacuum tubes used for amplification?

There are several types of vacuum tubes used for amplification, including triodes, tetrodes, and pentodes. Each type has a different number of electrodes and therefore, different characteristics. Triodes are the simplest and most commonly used for audio amplification, while tetrodes and pentodes are often used for high power applications.

5. How do vacuum tubes compare to other types of amplifiers?

Vacuum tubes have largely been replaced by solid-state amplifiers in modern technology. However, vacuum tubes still have a loyal following for their warm and natural sound, and are often preferred by musicians and audiophiles. They also have a unique distortion characteristic that is difficult to replicate with solid-state technology.

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