Schematic diagram and network analyzer plot (43kB)
The amplifier is quite typical two stage common source GaAsFET
amplifier. The FETs are
matched to 50 ohm using short sections of 50 ohm microstriplines and
SMD high-Q capacitors.
The input matching requires only a 4.7 pF parallel capacitor on the
gate. The output matching
circuits consist of 5mm series section of 50 ohm microstripline and
3.3 pF capacitor to ground.
The matching circuits of both stages are identical. The lengths of
the other 50 ohm microstripline
sections are uncritical because the impedance level is 50 ohms in those
connections.
The drain supplies and gate bias voltages are fed to the active devices
through high-Q SMD
inductors. There are 50 ohm resistors in series with the inductors
in gate bias supplies to
terminate the gates into resistive loads at low frequencies. This enhances
the stability of the
amplifier. There are 50 ohm resistors in series with 1nF capacitors
in the drain supplies for
the same purpose.
The amplifier is constructed on 1.6 mm thick double sided FR4 printed
circuit board. The
microstriplines are etched on one side of the board while the other
side is unetched and acts
as ground plane. The RF circuitry is surface assembled on the etched
side of the board.
On the unetched side there are the DC regulators etc. of the supply
circuitry, assembled
in 'ugly' style above the ground plane.
Photograph of the RF side of the amplifier. (887kB)
What does not show in the photograph is that there is a ground track
between the source
leads of the FETs. The middle lead of the FET is source as well as
the wide lead in the top
of the package. They are connected together under the FET and also
connected to the
ground plane by several pieces of soldered wire through the board.
The
proper RF grounding
of the source terminals is extremely critical.
It is also neccessary to take care of adequate
coolind since this amplifier dissipates something like 10 W power when
supplied from 13 V.
Schematic diagram of the supply circuitry. (16kB)
The GaAsFETs require negative gate voltage. The negative supply is generated
using
LT1054 (341kB pdf) charge pump IC. The negative supply
is regulated to -5 V with 7905 regulator.
The adjustable gate voltages for each stage are generated from the
-5 V regulated
voltage with resistive voltage dividers (20 kohm trimmer potentiometers).
There is a protection circuit which allows the drain voltage to be supplied
only if there is
negative voltage available. This protects the FET's against a possible
failure if the LT1054
does not start up and also delays the switching on of the drain voltage
until the gate voltages
are negative.
The drain supply is regulated to +8V but there are also 1.5 ohm series
resistors on the
drains of both FETs. Therefore the actual drain voltages, when biased,
are little above
+7 V.
Because the starting up of the LT1054 takes a while there is a separate
+TX input in the
amplifier to allow fast TX/RX switching. If there is voltage below
approx. 7 V in the +TX
input, the gates of both FETs are biased to -5V and the FETs are pinched
off. When a
positive voltage greater than 7 V is applied in the +TX input, the
FETs are biased to their
normal operating points.
Photograph of the DC side of the amplifier. (452kB)
This
photograph is not intended to be a
construction guide, it merely shows what I mean by 'ugly' construction.
Align the first stage first. The drain current should be measured during
alignment, it can be
conveniently done by measuring the voltage drop across the 1.5 ohm
resistor. Align the gate
voltage such that the drain current of the first FET is about 250 mA.
Then align the second stage
using similar procedure. The drain current of the second stage should
be approx. 500 mA
without any RF drive.
At 1 W RF output the drain current of the second stage drops to about
450 mA, but the first
stage current remains unchanged. The total current consumption is therefore
approx. 700 mA
at 1 W RF output.
There are no PCB etching patterns available for the amplifier at the
moment, sorry.
Petri Kotilainen OH3MCK
