Stability Issues with the Hybrid Cascode IF Amplifier
w7zoi (2 Dec 07)
It had to happen. Our IF amplifier (QST, December, 2007)
has four stages of gain, and three of them have a reactive element (choke)
in the output. In the remaining case, a transformer is present
that has some reactance. While these inductors are not purposefully
tuned, there are stray capacitances in the circuit that tune them somewhere.
Moreover, the inductors themselves have stray C that contributes to a self
resonant frequency (SRF) for the part. With this much gain and
this many tuned elements, stability problems should be suspected.
Sure enough, some examination revealed potential problems.
The original circuits we built used toroids in these positions.
These inductors were wound on -43 ferrite material, so were pretty lossy.
After all, this is the material that is often used to calm the EMI problems
that we face in our electronic world. The loss helped to stabilize
the amplifier. We were in the process of preparing a printed board
to help folks who want to use the Hybrid Cascode circuit in a receiver application.
In so doing, we had substituted some other RF chokes in the circuit.
The 120 uH inductors that were used in the SMT board were pretty good, even
if they were physically large. They had SRF of 14 MHz, indicating
a stray C of only about 1 pF. Still, I noticed the noise output
from a SMT board had quite a bit of peaking down below the nominal 9 MHz
amplifier frequency. Substitution of a lower L value inductor
in the differential amplifier output (L4) seemed to calm this problem.
Having seen a problem with a SMT PC board, I wondered if a problem would
exist with other variations where molded RF chokes were used.
So the early (no solder mask or silk screen) PC board was fit with the new
inductors. These 47 uH chokes had good enough Q, but had
SRF of 8.6 MHz. This corresponds to a parallel capacitance of
7 pF. Stray C adds to this to account for a strong noise peak
at 5 MHz that I saw in the amplifier output with AGC
turned off. If I put my finger or a metal tool on a critical point
in the circuit, the whole amplifier would go into oscillation at 5 MHz.
This did not happen when AGC was on.
Incidentally, the spectrum analyzer was extremely useful as a tool to study
this circuit. The analyzer was looking at the normal 50 Ohm output
from Q8 that would go to a product detector. The analyzer was
spanning from 0 to about 15 or 20 MHz, so showed the 9 MHz signal when one
was applied to the IF input. It would also show the wideband
noise drop when AGC is on and input signal is increased. Peaks
in the noise are readily observed.
Fortunately, the problem was fixed with relative ease. The dominant
modification was to add local negative feedback to the differential amplifier.
This stage used a pair of PNP transistors. Individual emitter
bias resistors were used to guarantee nearly equal current in each transistor.
The emitters were then tied to each other with a 0.1 uF capacitor.
The negative feedback was introduced as emitter degeneration and consisted
of a small value resistor in series with the 0.1 uF capacitor.
Even 10 Ohms did a lot to help things, and 22 was great.
The instability was further calmed by decreasing the gain in the IF amplifier.
This was realized by dropping R1 from 2.2 K to 1 K. The
result of these two changes is a noise output that is nearly flat when AGC
is off. The strong peak is no longer there. The overall
gain is still about 58 dB at 9 MHz. Output with AGC on is still
about -35 dBm or less, a reasonable level to drive a diode ring product detector
while maintaining very low IMD.
Incidentally, the input network driving Q2 was modified slightly.
The toroid specified in the QST paper was changed from 7.1 uH to 6.8 uH.
This allowed us to use an inexpensive choke in that slot.
The part we picked has a measured Q of about 70 at 9 MHz with SRF way up
at 111 MHz, so it performs well in this position. R7 is dropped
to 3.0 K to maintain a good input impedance match with the L-network using
the 6.8 uH choke.