FET amplifier for measuring LC circuits.

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Version 1

This amplifier can be used for measurements on LC circuits. The metal lid is removed for the photo.

The input of the amplifier is connected to the LC circuit.
The amplifiers input has the following properties:

a- High input resistance.
b- Low input capacitance (about 1.4 pF).
c- Low dielectric losses, because of the use of high quality insulation materials.

Because of this properties, the LC circuit is almost not loaded by the amplifier, so the Q will almost not reduce.

The output resistance of the amplifier is 50 Ohm.
If the amplifier output is not loaded, for instance it is connented to a 1 Mega Ohm oscilloscope input, then the amplifier gain is 1x, and the maximum output voltage is 8 Volt peak-peak.
If the output is loaded with 50 Ohm, then the gain is 0.5x and the maximal output voltage is 4 Volt peak-peak.
The gain is constant between 10 kHz and (at least) 10 MHz.

The amplifier output can e.g. be connected to:

- a oscilloscope
- a RF Voltmeter
- a RF Wattmeter
- Diode detector with voltmeter

Ampifier description:

The input signal enters the amplifier via a 0.3 pF input capacitor, toghether with the input capacitance of the FET (T1) this forms a voltage divider, the input signal is attenuated 17 times by this divider.

The 0.3 pF input capacitor is selfmade of two copperplates of 1 square cm at a distance of 3 mm.
By changing the distance between the plates we can adjust the gain of the amplifier.
The plates must have at least 1 cm distance from the surrounding grounded box.

The input signal enters the box via a 1mm copperwire, through a 10 mm hole in the box.
The wire is supported by a piece of polyethene, which is fixed with nylon screws.
The input amplifier (T1) is screened from the rest of the circuit.

Between the gate (input) of T1 and ground there is a 20 M.Ohm resistor.
But the input resistance of the amplifier is much higher then 20 M.Ohm, in theorie even 17² times higher  (so, 5780 M.Ohm), this is because over the 20 M.Ohm resistor is only 1/17th part of the input voltage.
In practice the input resistance will be lower then 5780 M.Ohm because of dielectric losses e.g. in the gate of the FET.

Transistor T2 is set to a gain of 17 times.
Or to be more precise -17 times, because this transistor is inverting the signal, but this is for the rest not important.
On the collector of T2 the amplitude is the same as the input amplitude of the amplifier.
The DC voltage on the collector of T2 must be about 6 to 7 Volt, if it is outside this range adjust the value of the 1K2 resistor at the base of T2.
T2 (BFR92A) is a very fast transistor (up to 5GHz) in SMD case, because of the high speed, T2 can give parasitic oscillation.
If this happens you better use a slower transistor like the BF199 (up to 500 MHz).

T3 and T4 form a buffer amplifier with a gain of 1x.
The amplifier is capable of driving a 50 Ohm load.

Amplifier version 2

The same amplifier is build once again, but with regard to version 1 with the following modifications:

- An aluminium case instead of a tinplate case, this gives less influence on circuit Q.

- The hole in the case for the input pin is increased to 13 mm (was 10 mm).

- The support for the input pin is now made of  polypropylene (was polyethene with a nylon screw).

- The support for the first capacitor plate is now made of polypropylene (was epoxy PCB material).

By means of these modifications I tried to reduce the dielectric losses in the amplifier.

The amplifier version 2. In an aluminium case of 112x62x30 mm.

Detail of the input stage.

In the next measurements I measured with both amplifiers the Q of  detectorunit1.
Also the Q is measured with both amplifiers parallel connected to the LC circuit.
The diode was in these measurements disconnected from the LC circuit.
When measuring "version 1",  I laid an aluminium plate on the amplifier, to increase circuit Q (see also lctest6 measurement 63).

Conclusions:
- Amplifier "version 2" gives a higher Q value, so version 2 gives less load (higher resistance) to the LC circuit.

- De Q factor with "version 1" and "version 2" parallel is almost equal to the Q measured with only "amplifier version 1".
This indicates that the input resistance (at that frequency) of "version 2" is almost infinite high, at least high enough to have almost no influence on measured Q.
 

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