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Old 30th September 2010, 08:25 AM   #1
seanvn is offline seanvn  Viet Nam
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Default What is the name of this topology?

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
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Old 30th September 2010, 11:53 AM   #2
AndrewT is online now AndrewT  Scotland
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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
regards Andrew T.
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Old 30th September 2010, 01:35 PM   #3
seanvn is offline seanvn  Viet Nam
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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?
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Old 30th September 2010, 01:42 PM   #4
seanvn is offline seanvn  Viet Nam
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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:

Last edited by seanvn; 30th September 2010 at 02:10 PM.
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Old 30th September 2010, 03:43 PM   #5
DRC is offline DRC  United Kingdom
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With C1 as an effective short, this is just a LTP variant - I think this is quite popular in IC amps.

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Old 30th September 2010, 04:57 PM   #6
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The gain of the second transistor is gm*R9 where gm=40*Ic and Ic is the DC operation point.

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Old 30th September 2010, 05:16 PM   #7
jcx is offline jcx  United States
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with a constant ref on the Q2 base it isn't working as a diff pair

could be considered a "folded" Rush Cascode
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Old 30th September 2010, 05:56 PM   #8
Mooly is offline Mooly  United Kingdom
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Originally Posted by jcx View Post
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.

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Old 30th September 2010, 06:26 PM   #9
DF96 is offline DF96  England
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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.
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Old 1st October 2010, 12:42 AM   #10
seanvn is offline seanvn  Viet Nam
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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.
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