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Circuit operation trans• nova implementation: – 13 – Hafler P1500 User Manual

Page 22

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The transistor Q1 is configured to operate as a switch which controls the current source, Q103, of the input differential
amp, Q6 and Q7. When Q1 is off, the emitter voltage is low turning off Q103. Timing of the Soft Start function is
controlled by the charging time of C29. Operation of Q1 is controlled by the THERMAL and SHORT protection circuits.
U1A and U1B are buffer amps configured as unity gain followers which feed the resistive network comprised of R24A,
R24B and R6 which attenuates the differential input at U2A and U2B. The output of U2A and U2B is taken to the inputs
of the differential amp Q6 and Q7, U2A is also configured as a DC servo integrator to null the input offset currents.
Transistor Q6 is the non-inverting input of the differential amp, taking the signal from U2A, the output is taken through
the current mirror, Q105 and Q104 where the signal is inverted, then feeds the negative driver Q11 and Q12. Transistor
Q7 is the inverting input of the differential amp, taking the signal from U2B. This output feeds the positive driver Q9
and Q10. The positive drivers Q9 and Q10, and the negative drivers Q11 and Q12 are cascoded stages which supply
drive current and voltage to the output devices. Class AB bias current is controlled by R136. The network R1 and C1
form the short loop feedback for the output stage. Global feedback is supplied by the network R10 and C8.

+18V

Positive Input

Buffer

U1A

Balanced Signal

Negative Input

Buffer

U1B

Level

Adjust

R24 A, B

Positive Buffer, DC

Offset Integrator

U2A, R11, C21,

C22

Negative Buffer

U2B

CMRR
Adjust

R8

Differential Amp

Q6, Q7

Current Mirror

Q104, Q105

Positive Driver

Cascode

Q9, Q10

+

Feedback

Network

Drive

Signal

Local

Feedback

Bias

Adjust

R136

Positive Output

Q4, Q115

B+

Output

B–

Negative Output

Q5, Q116

Negative Driver

Cascode

Q11, Q12

Current Source

Q103

Protection Switch,

Soft Start Delay

Q1, C29

–18V

CIRCUIT OPERATION

trans•

nova Implementation:

– 13 –

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