Model A5-24 Operation And Service Manual
A5-24 Manual 20 March 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 AC voltage gain of the amplifier is set by the load network,
T2, R19, L1. T2 is a wide band choke, and L1 provides high frequency compensation. The output
stages of the amplifier are common collector stages TR1 and TR6. R1 and R3 set a 10
0Ω
output
impedance for each transistor. When combined in T1, a 5
0Ω
output impedance results. Due to the
output impedance, the gain of the output stages is -6dB when the amplifier is loaded with 5
0
W. The
voltage gain of the entire amplifier is about 12dB, and it is capable of an output power of about 17
dBm into 5
0
W. At the nominal output power of 13dBm, 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. The
output may be floated by 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 boards include the input amplifier, the 2 x 12 way splitter, the power supply, and 12
output amplifier modules. The circuit of the main board is given on drawing no TBD. The input
amplifier is very similar to an output amplifier module, except variable gain is provided. The gain
controls on the VR1’s, which vary the transconductance of TR6 and TR7. C12 provides high
frequency compensation. The 5
0Ω
output impedance of the input amplifier provides a good match
to the passive splitter. The main board is supplied from the internal 24V DC power supply. A
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 circuits comprise RN1, D1 to D6, and TR11. If any of the output present transistors in the
output amplifiers are off, TR11 will turn on.
Performance Verification
Equipment Required
a)
Spectrum analyser and tracking generator, 1 MHz to 100 MHz.
b)
Sine wave signal generator or function generator, 10 MHz
c)
RF Power meter
d)
Low pass filter, Mini-circuits BLP 10.7
e)
50
Ω
BNC load
In addition, if input and output load and source impedance’s are to be checked, a network analyser
will be required.
