Agilent Technologies 33120A manual 740 R710 R717 740 R710 R711 R715 R716 R719 R720

Chapter 5 Theory of Operation

Output Amplifier

The block diagram shows four basic stages: dc amplifier, input differential amplifier, gain, and power output. The amplifier’s input differential amplifier stage and gain stage are symmetrical. The +AMP_IN and -AMP_IN inputs are both amplified through complementary amplifiers whose outputs are summed together at the input of the power output stage. Transistors Q701, Q702, Q704 and Q707 form the complementary input differential amplifiers. Q708 and Q705 are current sources which provide bias to the input differential amplifiers. Q709 and Q710 are emitter follower amplifiers used to couple the respective differential amplifier outputs to the gain stage transistors Q711 and Q715 which provide virtually all of the amplifiers open loop gain (~ x1000).

The power output stage is a wideband, class C buffer amplifier. Emitter followers Q714 and Q716 buffer the gain stage output from loading by the power output emitter follower transistors Q713 and Q718. Idle current bias for these power output transistors is set by the ratios of R732, R726 and transistor matching between Q713, Q714 and their equivalents in the other half of the stage: R734, R727 and Q718, Q716. Transistors Q712 and Q717 are current sources which provide bias to emitter followers Q714 and Q716 respectively.

The low frequency and dc performance of the amplifier is controlled by U702. This amplifier is used to sense the dc offset present at the +AMP_IN and -AMP_IN inputs and servo the output amplifier dc offset to zero volts; to the limit of U702’s own dc offset performance. U702 also provides a means to add a desired dc offset value into the output signal path through the x (-1) gain of the OUT_OFFSET signal.

The output amplifier employs a current feedback technique to set the closed-loop gain. The emitters of Q701 and Q702 are the virtual summing node points in the amplifier. Amplifier closed loop gain is controlled predominately by the following ratios:

2 ￿￿R￿740 ￿R710 ￿￿R717	and	2 ￿￿R￿740 ￿R710 ￿￿R711
R715 ￿R716		R719 ￿R720

Variable resistor R710 is used to match the gain through the high frequency feedback path (described above) and the dc feedback path summed through resistors R705, R706. The feedback signal current is injected into the amplifier through the emitters of Q701 and Q702 respectively.

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