What is the name of this topology?

seanvn

Member
2010-09-30 9:07 am
What is the name of this RC coupled differential amplifier topology.
I have tried using google but that was unsuccessfully. I just need the right keyword for it so that I can do some research.
I have used this circuit very successfully in a direct conversion receiver both as a RF and AF amplifier. It does seem to have very unique characteristics. I have seen a complementary feedback pair (CFP, Szilaki pair) version of it used in a high performance preamplifier. I just simplified it but it still works very well.
Can you tell me anything about it, including distortion levels, noise etc.

Sean O'Connor
amp.JPG
 
Q2 operates with two outputs. The Pr1 output is from the common emitter amplifier, gain is set by R8 & R4 ~1times.
The Pr2 output is from the common collector amplifier, the gain is ~1times, This is usually called an emitter follower.
The combined out of phase outputs from the single input is sometimes called a phase splitter. This is very common in tube (valve) amplifiers.

The second stage around BC108 is a common base amplifier. It works to very high frequency and the gain is set by R9 & R5. I think this is also ~1times. This is often referred to as a cascode.

You have two amplifiers:
phase splitter and cascode
 

seanvn

Member
2010-09-30 9:07 am
It is not so simple. The outputs at Pr1 and Pr2 are equal in magnitude but 180 degrees different in phase. The gain is about 150 when R7=0, it drops to 2 when R7=R4,R5.
If you make R7 much greater than R4,R5 then it ceases to be a differential amplifier.
I would suggest simulating it or building it to see how odd it is.
Anyone else any ideas?
 

seanvn

Member
2010-09-30 9:07 am
So basically T1 is not acting as acting as a phase splitter as you might easily imagine. There is actually voltage amplification at R8 (equal to that at R9). I should say I used different transistors just to see if transistor mismatch had any effect. The answer to that is no. Curious and curiouser.

The original circuit I took it from is here:
http://sound.westhost.com/project66.htm
 
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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
with a constant ref on the Q2 base it isn't working as a diff pair

Correct... it's visually drawn to look like an LTP but has nothing to do with that topology. The Rod Elliot circuit is however a differential amp.

The circuit here is just a two stage design with the second device as a common base amp which are very common in RF amps/tuners etc as a first stage to match to the aerial.

http://users.ece.gatech.edu/mleach/ece3050/notes/bjt/cbamp.pdf
 
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.
 

seanvn

Member
2010-09-30 9:07 am
Thanks for the information. So if you call the 2 transistors T1 and T1 the points T1-R4 and T2-R5 are virtual ground. Hence if C1 is large enough at the signal frequency the emitter load each transistor sees is (R4 parallel R7) and (R5 parallel R7) to a first approximation. Hence there will be over unity amplification at each collector.
Any ideas about Miller capacitance, noise level. A pointer to a research paper would be helpful but sometimes there are no papers! Hey, I know that.
 
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/6600367/description.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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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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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.