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Old 13th December 2004, 05:43 PM   #1
azira is offline azira  United States
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Default Single Stage Inverting or no?

I'm used to enhancement mode kinds of devices such as BJTs and Mosfets. JFets and Tubes are depletion mode devices correct?

Does a basic JFet/Tube single stage amplifier topology similar toe the Common Emitter, common source(that is a "Grounded Cathode" amp right?) topology amplify in phase? IN enhancement mode, this would be an inverting configuration.

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Old 13th December 2004, 06:22 PM   #2
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Grounded cathode topology inverts phase.
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Old 13th December 2004, 07:30 PM   #3
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Quote:
enhancement mode
You don't have a single electronics book around you, or what?
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Old 13th December 2004, 08:27 PM   #4
azira is offline azira  United States
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Quote:
Originally posted by analog_sa


You don't have a single electronics book around you, or what?
Sadly at this time no. I am a computer engineer by schooling graduated about 2 years ago. I took analog electronics classes as my electives because I've been interested in amplifiers since I was a kid. However, those classes chose to start with MOSFETS and BJTs and avoided JFETS. Right afterwards, I went to work as a digital logic designer and kicked off some real fun with this hobby using BJTs and Mosfets. In the past few months I just got a new job as a software designer and relocated to Seattle, all of my books and my hobby are sitting in boxes in my garage waiting for the day I can get the space to unpack them. I wish I could just design and play with amps for a living but unfortunatly, this job pays better. Sorry for the dumb question.
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Old 13th December 2004, 08:57 PM   #5
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There's a very nice thread over HERE with your answers!
All you need to know about TOOBZ!!
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Old 14th December 2004, 01:57 AM   #6
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Hi,

Another good place to start is to see how phase splitters go using one or two triodes:

VIVA CLAUDIO..........

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Old 14th December 2004, 09:53 AM   #7
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Default Re: Single Stage Inverting or no?

Pedroskova is correct that grounded cathode topology inverts phase.

I have a little time here, so I'll try to explain the best I can.

A grounded cathode amplifier (see the pdf article at www.tubecad.com ) commonly consists of a valve (triode/tetrode/pentode etc), with a load (commonly resistor) between the anode/plate and the power supply. A means of making the grid more negative than the cathode is generally necessary to prevent the cathode/grid forming a conducting diode (and losing the stage's high input Z). This is commonly done by placing a small valued resistor between cathode and ground. For now, this will be bypassed by a large capacitor, so little AC voltage appears across it (so we can ignore it for AC considerations).

So, we have a valve with stuff at its cathode (resistor||capacitor), which sets DC operating point, which we will ignore for AC. Also, there is a larger resistor (known as the plate load resistor) between the anode/plate and B+ (the power supply). The signal is taken off between the anode/plate and the plate load resistor.

Consider a triode for the moment (they're the simplest ). The hot cathode has a cloud of (negatively charged) electrons around it. The grid is biased negative with respect to the cathode, as such it repels the electrons. However, the plate carries a nice large positive voltage to attract the electrons towards it.

Should we apply a positive pulse to the grid, the grid becomes less negative. This makes the grid less effective in repelling electrons, thus more current flows through the triode. This increase in current must also flow through the plate load resistor. Therefore, the voltage across the plate load resistor must increase. As such, the voltage at the output node decreases.

Thus, a positive input voltage (into the grid) results in a negative output voltage (from the anode/plate). Hence, the stage inverts phase. A similar argument can be made for a negative input voltage but I won't (this post is way too long already).


I have explained it as I understand it at the moment, but if any more knowledgable members of the forum see errors, feel free to correct me.
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Old 14th December 2004, 03:56 PM   #8
azira is offline azira  United States
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Thanks everyone. I read the website and your comments... I think I understand. Depletion mode doesn't work like I thought it did at all. I think I need to unpack my books and go over JFets..
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