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Old 1st October 2010, 09:04 AM   #11
DRC is offline DRC  United Kingdom
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I still think the analysis will be, more or less, the same as an LTP where the tail resistor is R4/R5 in parellel and Re is R7/2. Assuming C1 is large enough, and of course, the dc analysis is different.

dc

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Old 1st October 2010, 10:35 AM   #12
AndrewT is offline AndrewT  Scotland
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no.
The separate tail sinks defeat the LTP action.

An LTP uses the constant current draw of the tail to influence how the other half amplifies it's input signal.
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Old 1st October 2010, 10:39 AM   #13
DRC is offline DRC  United Kingdom
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Quote:
An LTP uses the constant current draw of the tail to influence how the other half amplifies it's input signal.
thats what R7 + C1 do !
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Old 1st October 2010, 11:59 AM   #14
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Quote:
Originally Posted by DF96 View Post
This is an emitter-coupled amplifier. The valve analogue is called the cathode-coupled amp. It is a form of differential amp. Differences from a normal LTP are: not necessarily balanced, often quite a 'short' tail, often AC coupling, not necessarily the same transistor type. To a first approximation all BJT behave in exactly the same way: exponential response. That is why changing the transistor had little effect. A normal LTP uses a matched pair in order to maintain DC balance.

The ECA will typically have lower even-order distortion than a common-emitter amp. As the output is in phase with the input it might have greater risk of oscillation due to capacitive feedback.
I can also call that emitter coupled cascode (another form of the good known normal cascode or folded cascode).
check out US Patent 6,600,367
http://www.patentstorm.us/patents/66...scription.html
http://www.pat2pdf.org/patents/pat6600367.pdf
and
Information Bridge: DOE Scientific and Technical Information - Sponsored by OSTI

This topology I get often automaticly, when I transform a power amp from the non inverted mode to an inverted mode.

But an additional fact is of interest:
After replace R4 and R5 each through current sources (two independend current sources instead commonly used one pcs. for both halves) and R7 through a potentiometer, you get the possibility for easy variation of the open loop gain/damping factor from your whole power amp stage.

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Old 1st October 2010, 06:30 PM   #15
DF96 is offline DF96  England
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Quote:
no.
The separate tail sinks defeat the LTP action.
Get rid of R7 and you have a short-tail pair. The tail is just R4 and R5 in parallel. Obviously this doesn't go down to DC because of the capacitor.
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Old 2nd October 2010, 12:06 AM   #16
cbdb is offline cbdb  Canada
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A diff amp needs a diff input. T2s base is ac grounded. You have 2 outs, the first from a CE amp, the second from a 2 stage amp (cascode) CC then CB (common base).
Thats the way it should be analized, not as a diff amp which is possible but thats over complicating things for no reason. (draw the second transistor rotated 90 degrees clockwise, the usual way to draw a CB amp, and maybe it will be easier to understand)

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Old 2nd October 2010, 11:46 AM   #17
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Quote:
Originally Posted by cbdb View Post
A diff amp needs a diff input. T2s base is ac grounded. You have 2 outs, the first from a CE amp, the second from a 2 stage amp (cascode) CC then CB (common base).
Thats the way it should be analized, not as a diff amp which is possible but thats over complicating things for no reason. (draw the second transistor rotated 90 degrees clockwise, the usual way to draw a CB amp, and maybe it will be easier to understand)
Yes, the kind of drawing often determines, how well you understand the right working of a certainly circuit.
Independend of this - despite of the fact of grounding the inverted input it is still a diff amp - then the different between non inverted input and GND.
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