Common base with base series resistor

[Andrew Holme]

One often sees a small resistor (e.g. 22 - 100 ohms) inserted in series with
the base in VHF/UHF/Microwave common base amplifier circuits. I believe
this aids stability but I don't understand how it works. I'd be grateful if
someone could explain.

I'm building an 80 MHz crystal oscillator based on [1] and I had a spurious
~ 1.3 GHz parasitic oscillation which went away when I increase the base
series resistance of the top cascode transistor from 22 to 100 ohms. I
haven't tried values in-between yet. I'm using BFM520 instead of the
obsolete and unobtainable device specified in [1].

I'm also curious why one would need a transistor with such a high fT
(several GHz) for a 100 MHz oscillator. Is it because they come with very
low junction capacitances and low noise figures, or do we actually need the
fT as well?

TIA

1: http://uwsdr.berlios.de/A New LNXO Topology.pdf

 

[Andrew Holme]

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One often sees a small resistor (e.g. 22 - 100 ohms) inserted in series
with
the base in VHF/UHF/Microwave common base amplifier circuits. I believe
this aids stability but I don't understand how it works. I'd be grateful
if
someone could explain.

I'm building an 80 MHz crystal oscillator based on [1] and I had a
spurious
~ 1.3 GHz parasitic oscillation which went away when I increase the base
series resistance of the top cascode transistor from 22 to 100 ohms. I
haven't tried values in-between yet. I'm using BFM520 instead of the
obsolete and unobtainable device specified in [1].

I'm also curious why one would need a transistor with such a high fT
(several GHz) for a 100 MHz oscillator. Is it because they come with very
low junction capacitances and low noise figures, or do we actually need
the
fT as well?

TIA

1:http://uwsdr.berlios.de/A New LNXO Topology.pdf

Hello Andrew,

Hello,

Transistors with a capacitive loaded emitter will oscillate very
quickly with some base inductance. The feedback path is via the BE
diffusion capacitance. I use this property frequently in oscillator
design when design frequency is not far below ft.

If you have a simulation program, "make" a circuit with grounded
collector. Drive the base with an AC current source and "measure" the
voltage. When you do an AC analysis in PSPICE, you can select 1A
current. The voltage across the current source equals the input
impedance of the transistor. When the emitter is grounded for AC, you
will measure dissipative impedance.

Now, play with the emitter capacitance (provide a DC path via a
current source, so the circuit has DC bias). You will notice negative
real part in input impedance. Just a small inductance (lead
inductance) can be sufficient to make a nice oscillator. Adding some
base resistance kills the negative resistance and avoids parasitic
oscillation due to parasitic inductance in the base circuit.

When you make a cascode or common base circuit, there can be frequency
ranges where the emitter sees capacitive impedance. To avoid parasitic
oscillation, the base resistor may help. Ferrite does also a good job
(as this doesn't affect noise behavior mostly). The difficulty with
ferrite is to know what you have, so the resistor is a good
alternative.

Regarding ft. High ft will not always result in low noise. Oscillator
phase noise close to the carrier is caused by LF noise in the
transistor (and current carrying resistors).

Using high ft transistors can be useful in isolating amplifiers (to
avoid frequency drift under varying load). Low Cce and Ccb will result
in less feedback in case of a common base amplifier (cascode
circuit).

Also high ft transistors behave more ideally, so design can be easier
(except for parasitic oscillation).

Best regards and good luck with the oscillator,

Wim
PA3DJS
www.tetech.nl
In case of PM, please remove abc first.


Wim, thanks for the excellent explanation and also for the interesting
simulator experiment, which I look forward to trying in LTSpice.