FET amplifier for measuring LC circuits.

Back to the index

Version 1

Figure 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 connected 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:

- An oscilloscope
- RF voltmeter
- RF wattmeter
- Diode detector with voltmeter

Figure 2: Circuit diagram of the FET amplifier (see also the updates at the bottom of this page)

Circuit description:

The input signal enters the amplifier via a 0.3 pF input capacitor, together 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 self-made 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 17 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 or 10K 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.

Figure 3: the amplifier version 2.  In an aluminium case of 112x62x30 mm.

Figure 4.

Figure 5: detail of the input stage.

Figure 6.

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).

Amplifier version Q
600 kHz
900 kHz
1200 kHz
1500 kHz
Version 1 1111 1046 889 731
Version 2 1111 1125 952 802
Version 1 and version 2
1090 1046 882 731
Q factor of detector unit 1
Measured with different measuring amplifiers.

- 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.

Update 1

Here a part of the circuit diagram of figure 2:

Figure 7:    12 V power circuit for the FET amplifier.

The practice has shown that the input diode (BYV10-40), easily gets defective when connecting the supply voltage (the diode gets an internal short-circuit).
This may be caused by static charge exceeding the maximum reverse voltage of the diode.
Or by too high peak current when charging the 100 μF capacitor at the input of the 7812.

To solve both problems, here some modifications, in figure 7 drawn in red.
- At the power supply input, a 100 nF capacitor to ground is added (removes static charge).
- The value of the 100 μF capacitor is reduces to 1 μF (reduces peak current at power-on)

The current consumption of the FET amplifier, inclusive 7812 voltage regulator is about 43 mA.

Update 2

When using the 7812 voltage regulator, at least 15 volt input voltage is needed.
This voltage can well be supplied by a non-stabilized 12 V means-adaptor,
these adaptors deliver 12 V at maximum supply current, but at light loads much more voltage, for instance about 17 volt DC.

If you want to power the circuit also from a 12V rechargeable battery, you however have not so much voltage available.
Depending on the charging condition of the battery, 12 to 13.8 volt.
By using a low-drop regulator it is possible to get a 12 volt stabilized voltage at these low battery voltages.
The reasons for powering the circuit from a battery can be:
- For doing measurements in the field, where no means voltage is available.
- To prevent interference from the means.
- To prevent an unintended connection between circuit and earth, via the internal capacitance in the means adaptor.

Figure 8: power circuit for the FET amplifier with low-drop voltage regulator.

As low-drop voltage regulator I used the LM2940T-12 (LM2940_datasheet) , because I had one laying around.
You can also use the LM2940CT-12, which is a more common type.
The minimum voltage between input and output is depending on load current.
But with the 40 mA the FET amplifier uses, there is at least 50 mV necessary between input and output.
So at only 12.05 volt battery voltage, we still have a stabilized 12 Volt output voltage.
When the battery voltage drops below 12 volt, the output of the LM2940 will follow at 50 mV below that.

If the voltage regulator has enough cooling, the input voltage may be increased to a maximum of about 29 volt.
Above 30 volt input voltage,  the regulator is switching off the output voltage.
The FET amplifier inclusive LM2940T-12 uses about 50 mA.

One nice property of this voltage regulator is, that it can withstand negative input voltages.
So if you accidentally connect the input voltage with wrong polarity, it can do no harm.
Therefore there is no need for a diode to protect the voltage regulator for negative input voltages.

The capacitor at the output of the LM2940T-12 must have an internal resistance (ESR value) between 0.1 Ω and 1 Ω.
The 47 μF/25V capacitor I used, has a measured resistance value of 0.3 Ω, so it was useable.

Figure 9:  this picture shows the frequency response of the amplifier (version 2) between 0 and 50 MHz.
The - 3 dB point is at 14 MHz.

Back to the index