HP TM 11-6625-2779-14&P manual Meter AMPLIFIER. Schematic No, DETECTOR. Schematic No

Model 3555B

identical in operation, only the first will be discussed in detail.

4-27. Referring to Figure 7-4,the signal is applied to the assembly through pin 22. If C MSG is selected the signal is first attenuated by A4R1, A4R2 and A4R3A. Potentiometer A4R3A is for C MSG level adjustment for 0dB at 1kHz. The signal is then applied to the first in a series of amplifiers. The first amplifier consists of A4Q1 through A4Q4. Differential amplifier A4Q1 and A4Q2 amplifies the signal and applies it to A4Q3 and A4Q4. The emitter circuit of A4Q4 provides two feedback signals, positive feedback through A4R8 and A4C4 to the base of A4Q1 and negative feedback to the base of A4Q2. The gain of this amplifier is controlled by the ratio of the value of A4R10 to the value of A4R9. For example, increasing the value of A4R9 would increase the negative feedback and reduce the amplifier gain. Gain can be calculated by the equation:

A4R10 Gain = 1 + A4R9

Positive feedback to the base of A4Q1 determines the frequency response of this amplifier and is controlled by the value of A4C4 and A4R8. All five of the amplifiers are used in C Message weighting.

4-28. The Program weighting filter utilizes only amplifiers No. 2 and No. 3 as shown in Figure 7-1.These amplifiers are identical to the one described in the preceeding paragraph except for the value of the positive feedback utilized for shaping and the negative feedback used for gain control. This negative feedback is modified by resistance in the feedback divider at the base of A4Q12. Transistors A4Q5 and A4Q6 provide additional gain required for Program weighting. Potentiometer A4R3B is used for PROG level adjustment at 1kHz.

Section IV

4-29. The 3kHz FLAT and 15kHz FLAT weighting filters utilize only amplifier as indicated in Figure 7-1.The only difference between these two active filters is in the positive feedback used for shaping and in the

negative feedback used for gain. The negative feedback is altered by adding resistance to the feedback divider at the base of A4Q12.

4-30. METER AMPLIFIER. (Schematic No. 4)

4-31. The meter amplifier consists of A3Q6 through A3Q10. The signal is first amplified by differential amplifier A3Q6 and A3Q7. The signal is taken from the collector of A3Q6 and then amplified by A3Q9 and A3Q10. Transistor A3Q8 provides isolation between A3Q7 and A3Q9 to prevent undesired feedback. Two signals are taken from A3Q10. The collector circuit supplies a signal to the DIAL/AC MON jacks for the purpose of listening to the measured signal. The emitter circuit of A2Q10 provides a drive signal for the detector circuit.

4-32. DETECTOR. (Schematic No. 4)

4-33. The detector is a class B rms detector which combines the features of an average detector and a peak detector. When the average detected signals and the peak detected signals are combined in the proper proportion an equivalent rms response is produced.

4-34. First consider the average detection in this circuit. (See Figure 7-5). Transistors A3Q12-A3Q13 and A3Q15-A3Q16 are functionally symmetrical. This means that A3Q14 and A3Q17 are driven by the same signal. When the signal at the base of A3017 and A3Q14 goes negative, A3Q! 4 turns on and A3Q17 turns off. No current will flow through the meter. On the positive half cycle A3Q14 turns off and A3Q17 turns on. The current paths for the average detector are shown in Figure 4-4.

Figure 4-7. Simplified Peak Detection

4-7