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Figure 3. Swept frequency response of the oscilloscope preamplifier (Note: the marker wfti set at the half power point, or — 3 dB J

While device capacitance between terminals affects the overall amplifier gain, the capacitance that exists between input and output terminals has the greatest effect. This condition is known as the "Miller Effect/' and results in the effective capacitance being magnified by the voltage gain of the amplifier.

In summary, the ground plane should connect all unused areas of the pattern side of the printed circuit board. The ground plane then provides a tow resistance, low inductance common return path for all signal and power returns, and reduces stray signal pick up.

Other Considerations

This circuit uses positive and negative DC voltages. To keep the supply voltage free of ripple and noise, bypass each power supply lead to ground. Solder the bypass capacitor as near as possible to integrated circuit pins. A 0.1 |-lF capacitor normally suffices. However. in high frequency and/or high gain circuits, a parallel combination of a 1.0 |iF and 470 pF performs a suitable bypass.

The circuit easily mounted on a 4 cm x 5 cm perforated board that had a sufficiently large ground plane. 1 used no enclosure for this circuit. There are no IC sockets, and all components soldered directly to the ^perf* board.

Single-point grounding is recommended in cases where point-to-point connections are used, or where a ground plane is not used. The input signal common, the load signal common, and the power supply common should be connected to the same physical point. This eliminates ground loops or common current paths which may cause signal modulation, distortion, or unwanted feedback.

Preamp Performance

I tested the oscilloscope preamplifier with a sweep generator and spectrum analyzer over a frequency range of0.5 MHz to 164.4 MHz. Figure 3 shows the results.

As designed, the oscilloscope preamplifier provided a voltage gain of approximately 10 (±0.1 dB) from 0.5 MHz to 50 MHz. It actually only roiled off - 3 dB at 54.10 MHz, and so exceeded specifications as shown in Figure 3. E3

Pro pa g a tion by Jim Gray W1XU

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