Miller Wide Band

Figure 2. The preamp circuit board.

iional gain is needed. If you intend to use this circuit as a mike preamp around high RF power, some RFI-proofing (limiting the RF end of the response curve with ferrite beads and RF bypass capacitors) would be well advised.

Construction can be done on a perf board, keeping all leads as short as possible. The specifications and parts list are shown below,


Usable frequency range: AF from 50 Hz to RF over 100 MHz, with a gain of +35dB at 100 kHz, +30dB at 10 MHz, and +17dB at 100 MHz. The noise is under 10 microvolts across 50 ohms {greater with a higher terminating impedance). The output is 100 millivolts or better into 50 ohms and 500 millivolts or better into 600 ohms.

The power supply requirements are 12-15 VDC at about 10 mA. The output begins to drop with less than a I2-V supply, and audio linearity begins to suffer below 10 V at maximum input. The input should be kept below about -30 dBm or 30 millivolts (.03 V mis). Noticable flattening of the waveform begins with inputs greater than about -20 dBm or 100 millivolts (0.1 V mis).

Parts placement is not particularly critical, as long as the input and output arc well separated. It may be laid out basically, as the schematic diagram (see Figure I) is drawn with a ground bus around the perimeter of the board, R6 is an optional Ik-gain control that may either be placed in parallel with, or used instead of, fixed resistor R2 to allow for variable gain output. You can also reduce the preamp's gain by inserting an appropriate input or output fixed resistive pad.

Parts List

a( 2

1 -fiF 35 V tantalum capacitors

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