[time-nuts] Advice on 10 MHz isolation/distribution amplifier (Clay)
bruce.griffiths at xtra.co.nz
Thu Feb 11 23:12:29 UTC 2010
life speed wrote:
> Message: 2
> Date: Thu, 11 Feb 2010 07:54:40 -0500
> From: Bob Camp<lists at cq.nu>
> Implementing that circuit without using a hybrid would be a bit of a challenge.
> Message: 6
> Date: Fri, 12 Feb 2010 04:09:08 +1300
> From: Bruce Griffiths<bruce.griffiths at xtra.co.nz>
> Yes implementing an exact copy without using a hybrid would be difficult.
> However for 10MHz use, its probably not too difficult since that
> isolation amplifier is intended for a 100MHz signal and the requirement
> is for 10MHz operation.
> If the transistor ft's are reduced by a factor of 10 or so it shouldn't
> be too much of a problem.
> At 10MHz 2N3906 and 2N3904 transistors should suffice.
> Hi Bruce,
> Thanks for the tips. I've been trying to follow the circuits you posted. The first one, in .PNG format, looks like a common-base complementary (push-pull) stage followed by a common-emitter complementary stage to provide the low impedance output.
> The second circut in .GIF fromat I am having a bit more trouble understanding. I see that V6, 7 are at the outputs and just used for to simulate isolation. V1 is the input?
V1 is indeed the input.
> Are Q5,6 used to set the bias point of Q4? Are V2,3,4 just there to bias the transistors for simulation purposes, and this would be accomplished another way in a real implementation?
The output (collectors of Q5, Q6 emitter of Q4) of the input amplifier
sets the dc voltage at the inputs ( Q1 base, Q7 base respectively) of
the output amplifiers.
The circuit consists of a unity gain input amplifier (Q4, Q5, Q6) that
drives a pair of output amplifiers (Q1, Q2, Q3 and Q7, Q8, Q9
respectively) each with a gain of 2x (6dB).
The input amplifier is essentially a white emitter follower with a
complementary symmetry output stage (shown in transistor electronics
books from the 1960's) where an input CE transistor drives a
complementary pair of CE transistors with feedback from the common
collectors of the 2 output transistors to the input transistor emitter.
In effect its merely a very simple unity gain opamp. Its usually best to
ensure that the CE output stage pair provide the dominant open loop
pole. Using a higher ft (2 to 3x) input transistor than the output pair
is the usual way of ensuring this.
The output stages can be viewed as simple 3 transistor current feedback
opamps with a nominal gain of about 2x (6dB).
The output stage gain being adjusted in this case for an overall gain of
0dB when driving a 50 ohm load.
The 47 ohm resistors in series with the outputs match the output
impedance to that of a 50 ohm cable.
With a closed loop gain of 2 ensuring that the ft of the input
transistor is greater (>2x) than that of the output stage transistors is
Both output transistors contribute to the RF output signal.
The npn output transistor are also used to set the operating current of
the output stage.
The resistor in series with the npn output transistor emitter is
bypassed for RF so that the full gain of this transistor is available at RF.
Using a complementary symmetry output stage allows the dc collector
current of the output stage to be reduced to about half that required if
the npn output stage transistor were merely acting as a fixed current
Yes the 1.7V dc sources are only included for the simulation.
I just wanted to illustrate the principles without getting into too much
detail in that post.
In practice one could either use a LED or a resistive voltage divider
buffered by a pnp emitter follower (either method provides a degree of
temperature compensation for Vbe tempco of the npn output transistors) .
Either one uses independent biasing for each npn CE device, or elaborate
RC filtering (at least an independent 2 stage filter for each transistor
) to avoid degrading the RF reverse isolation via the RF impedance of
the common 1.7V bias circuit.
As noted in a later post, using unity gain output amplifiers with a 2x
gain input stage allows the total dc current to be reduced below that of
when the input stage has unity gain and the output stages have voltage
gain of 2X.
> Please explain the comment regarding the hybrid. Are you and Bob referring to a 90 degree hybrid coupler, or other quadrature method like a transmission line transformer? What would be the purpose of such a device?
> Would it be too much to ask for a description of these circuits? I suppose we all have our areas of expertise, and transistor isolation amps are somewhat new to me.
> Thanks again for all the help.
> PS - yes, the OCXO is vibe isolated. And you are certainly correct about long runs of single-ended coax being susceptible to noise. The system designer has accepted this and allowed for some degradation. But I will look into the practicality of implementing a differential line for the long run of 10 MHz cable. However, I will still need to implement traditional coaxial isolated 10 MHz outputs.
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