Common collector amplifier and amplitude modulation

Could some electronics guru here shed some light on this Could a single stage common collector amplifier exhibit amplitude modulation ?

I have the SPICE model of a simple common collector amplifier consisting of two base bias resistors, a collector resistor and an input and a output DC blocking capacitor, as well as a source and a load resistor of 50.0 Ohm. The BJT collector is connected to Vcc via a RFC coil. I have tried input signal frequencies from 100 MHz to 750 MHz, with SPICE transient analysis. Each time, I have seen a output signal envelope, at the same spot on the time axis. I have tried several common RF|microwave transistors as BFR92A, BFS17, BFQ67W and HFA3134, but the bproblem is always there.

All hints and suggestions are welcome - thanks in advance.

Reply to
dakupoto
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Constant-voltage base biasing is fine at audio or low RF frequencies but it's a lousy way to bias a high-frequency RF single-transistor amp.

Please see e.g.

for how to bias RF amps at those frequencies, properly...

Reply to
bitrex

That is to say it sounds like you're trying to use a variant of this circuit:

to amplify 750MHz. the answer is fuggedaboutit.

Reply to
bitrex

Can you draw me a picture. that doesn't sound particularly common collector.

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Reply to
Jasen Betts

Post the Spice sim? .asc for LT Spice.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

I have seen this paper in the past, and in fact I do have a copy on a CD. The main issue is there is no indication as to how the the passive device component values(in particular resistors) are computed. I remember in the section on current mirror based biasing the author writes something like "..with appropriate choice of R1, R2...". So how are values of R1, R2 calculated anyway ?

Reply to
dakupoto

I use HSpice and Ngspice, both of which allow text netlists, but the circuit is so simple that I have given a description below: Vcc 15 Volt VCC -> Collector terminal 1.0uH Emitter terminal -> ground - RE -375 Ohm Vcc -> intermediate node - RB1 - 3750 Ohm Intermediate node -< ground - RB2 - 595 Ohm Intermediate node -> base terminal - RFC coil 1.0uH Both DC blocking capacitor(input, output) are 10 uF Input signal amplitude is 1.5 Volt, the frequency has been varied between 100 MHz and 750 MHz, mostly around 250 MHz

Reply to
dakupoto

I do not have any collector resistor, and the output is taken (via a DC blocking capacitor) from the transistor emitter terminal. Please see my response to John Larkin's message.

Reply to
dakupoto

Err... isn't a "common collector amplifier" an emitter follower? And....isn't the circuit you are talking about a common emitter amplifier (base in, collector out)?

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Reply to
Robert Baer

Your RFC makes it no longer a common emitter amp. You fail to say what you actually mean by 'exhibit amplitude modulation'.

NT

Reply to
tabbypurr

For it to be common collector (also called emitter follower) the collector has to be common to both input and load circuits or put another way has to be at AC ground if both load and input are ground referenced. Having a 1uH inductor in series with the collector completely changes things.

The reactance of 1uH at 750MHz is 4700 ohms - think what effect that might have.

And why do you have 1uH in series with the base?

I think changing both to 1nH (one nano-henry) would be better simulation of real world stray wiring inductance.

piglet

Reply to
Piglet

To make it work as a common collector amplifier, you need to provide a direct RF path to ground from the collector with a by-pass capacitor. The RF choke spoils the ampifier.

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-TV
Reply to
Tauno Voipio

Choose R1 and R2 to put the base voltage (and hence the emitter voltage plus Vbe drop) at the quiescent bias point you want for the signal handling you want.

In your example 3750 and 595 ohms from 15V puts the base at 2.05V and emitter about 0.7V lower. That means it will be incapable of handling the low going part of your 1.5V ac input as the transistor will be cut off before the low going peak and output will be clipped.

Better is to raise the bias so transistor remains conducting for all expected inputs. The usual rule of thumb is mid-point. i.e. emitter at

7.5V thus base at 8.2V so one could use 680 and 820 ohms.

piglet

Reply to
Piglet

He hasn't posted his spice deck, but I suspect the turn on transient through one of his chokes will cause a slow oscillation, quenching the Ic to zero periodically and thus modulate his output with a low frequency sine.

(I happened to play with sthg similar earlier)

Reply to
Johann Klammer

Too many words! How about a screen shot or a sketch?

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

I hope that I understood his description right:

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Reply to
Tauno Voipio

You got the input signal going to the base with L in the bias feed, I read the OP description as having the L in series with the base so signal input also went via the L.

I agree it unclear which is the right interpretation?

piglet

Reply to
Piglet

There is no description where the feed capacitor connects near the base. I tried to pick the less bad interpretation.

A 1uH series inductor strangles the signal pretty well.

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-TV
Reply to
Tauno Voipio

We can't help you if you don't show us what you are doing.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

Hmm I'm thinking you need to down load LTspice and post *.asc schematics.

George H.

Reply to
George Herold

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