Model A5-32 Operation And Service Manual
A5-32 Manual 29 April 2002
Page 6 of 10
Circuit Descriptions
Output Amplifier
The output amplifier is a differential amplifier based on a common emitter, common base, and
common collector cascade circuit. TR7 and TR8 are the common emitter input stages. The emitter
resistors R11 and R23, and R12 set the transconductance. A feedback circuit comprising TR4/TR5
and TR9/TR10 sets the DC collector current in each transistor. The voltage across the emitter
resistors is set equal to a 2.5V reference voltage fed to the amplifier from the "BIAS" pin. The
active control of DC current also reduces the current fluctuations due to 1/f noise in the transistors.
This reduces the phase noise.
TR2 and TR3 are the common base stages. These provide most of the reverse isolation in the
amplifier.
The load network, T2, R19, L1, sets the AC voltage gain of the amplifier. T2 is a wide band choke,
and L1 provides high frequency compensation.
The output stage of the amplifier is common collector stages TR1 and TR6. R1 and R3 set 100
Ω
output impedance for each transistor. When combined in T1, a 50
Ω
output impedance results. Due
to the output impedance, the gain of the output stages is –6 dB when the amplifier is loaded with
50
Ω
.
The voltage gain of the entire amplifier is about 12 dB, and it is capable of an output power of
about 17 dBm into 50
Ω
. At the nominal output power of 13 dBm, the second harmonic distortion is
less than -60 dBc due to the differential design. Third harmonic distortion is less than -40 dBc. The
bandwidth is +/-1 dB from 2 MHz to 100 MHz. The gain falls by a further 1 dB at 1 MHz.
A tuned version of the output amplifier is possible by replacing T2 by a tuned transformer T4, and
by adding C21. This reduces harmonic distortion further, and also reduces integrated wide band
noise.
Cutting a track, enabling an isolated output option to be available on some products. The network
associated with D1 rectifies a sample of the AC output and turns on TR11. This provides an output
monitor point on J2. The amplifier is powered from +18V DC and +12V DC supplies. The power
consumption is about 100 mA at 1 V and 40 mA at 12V.
Mother Board
The main board includes the input amplifier, the 12-way splitter, the power supply, and 12 output
amplifier modules.
The input amplifier is very similar to an output amplifier module, except variable gain is provided.
The gain control is VR1, which varies the transconductance of TR6 and TR7. C12 provides high
frequency compensation. The 50
Ω
output impedance of the input amplifier provides a good match
to the passive splitter, PS1.
The main board is supplied from a 24V DC supply. An off board regulator provides 18V. R41 and
R42 set the voltage of the 18V supply.
U3 is the 12V regulator. U1 provides the 2.5V bias supply for the input amplifier and all the output
amplifiers.
The output fail circuit comprises RN1, D1 to D6, and TR11. If any of the output present transistors
in the output amplifiers are off, TR11 will turn on.
