QRP design- it is very specific design, where ham wants to use as little
parts as possible. The amplifier gives the possibility because
it can be used twice- like RF- PA and like RF- RX- Amplifier.
Technical Data for the amplifier:
Supply Voltage: 12-V
Gain: near 15 (Pout/Pin) (at 28.0- MHz) – 20 (Pout/Pin) (at 1.8- MHz)
Input/Output Impedance: Close to 100- Ohm (amplifier was designed to
work with 75-Ohm Coaxial Cable). Figure 1 (p.71) shows schematic of the amplifier.
The schematic of the Universal RF Amplifier
is simple. It is typical Push- Pull. For properly work of the
amplifier it needs to install the same collector current (with
R3 and R6). Value for the current is 15- 25-mA.
Transistors VT1 and VT2 should be a matched pair (have
the same gain at third different collector currents- 10, 50 and
R4C3 and R5C4 are lowered the gain but rise the stability.
Sometimes (when the Amplifier is used at range 1.8- 14.0- MHz
and matched antenna is used) it is possible do not install them.
Gain is raised up to 25 (Pout/Pin) at this case. With the transistors
(KT606A, DATA for the transistors at: http://www.antentop.org/008/bip008.htm).
The amplifier work out up to 1-Wtts output power. However, do
not drive into the amplifier more the 50-mWtts because the output
waveform signal may be distorted.
2 (p.72) shows the commutation between a QRP- transceiver
and the amplifier. For switching RX/TX a two small relays are used. At TX
mode the Amplifier should be matched with a QRP- Transceiver.
Separate circuit (like L or pi- circuit) for matching transceiver
with antenna is used for each band. At RX mode the matching circuit
is used for filtering of the input signal. At my design the Amplifier
at RX- mode was loaded on to balanced diode mixer.
All capacitors should have low losses at RF. All transformers
are wound by pair of twisted
wires (two twist on 1-cm length) in diameter 0.3- mm (29- AWG).
Core- OD- 7…15 mm, height- 3- 7- mm, permeability-
400… 600. Numbers of winding for each transformers
is 20. It is uniform winding along all length of the core. However,
for best efficiency of the amplifier the number of winding for
each transformer should be experimentally chosen.
Design: The amplifier is assembled on a PCB by sizes 40x 50-mm.
Two holes for VT1 and VT2 are drilled at the PCB. The transistors
are installed on aluminum plate with sizes equal to PCB. Parts-
resistors/transformers/capacitors are installed on the transistors
pins and small circles cut on the PCB. Try to keep the leads from
the parts as short as possible. Relays are installed near output
and mixer of the QRP- Transceiver and connected with the Amplifier
with a thin Coaxial Cable.
73! Igor Grigorov, va3znw