Simple HF amplifier for QRSS beacon

This is 5 minutes soldering project. There is no schematic, just simple circuit, something between “dead bug” and conventional PCB design. I did this in two steps. First attempt was complete failure. Local OM told me that any power mosfet can operate as power amplifier for HF. Ok, maybe not just any. I tested with couple of them including IRFZ24N. Finaly I took “proper”  MOSFET for higher frequency and the circuit worked pretty good:

Simple output stage for QRSS beacon assembled in "mixed" technology

Simple output stage for QRSS beacon assembled in “mixed” technology

I will briefly describe this quick and simple project and I hope readers would enjoy it.

The prototyping circuit for TO220 power mosfets was (this did not worked as expected):

Schematic for preliminary testing

Schematic for preliminary testing

The result was not good. All FETs I tested had top frequency limit at less than 3MHz. My QRSS beacon operates at 7MHz and I wanted at least 100mW output from 0dBm input at that frequency. Anopther problem with such simple circuit is variable bias when supply voltage varies. The second solution solved both problems. First I took “proper” MOSFET from the final stage pre-driver used in old NMT base station. Another improvement was bias voltage stabilisation with Zener diode. The final schematic is:

Proper schematic for ultra simple QRSS amplifier

Proper schematic for ultra simple QRSS amplifier

It is important to attach the MOSFET to the heatsink. It should operate with bias currents around Id0=200mA. When supplied with 20V it will dissipate around 3-4W, which is enoug to fry it without the heat sink. The connection diagram with RF power FET housing is:

Principle connection diagram for power MOSFET

Principle connection diagram for power MOSFET

The component values are: Rz = 820 OHM, Multiturn potenciometer = 5k, L1 = 1uH RF choke, L2 = 100uH toroid coil, C1 = 22nF, C2 = 68nF (both C0G ceramics), Zener diode = 5,6V, Transistor in my case was BLF542. The power supply was 20V for all tests (except for the distorted oscillogram, where supply voltage was 11V).

The soldered circuit looks something like this:

Finished circuit with heatsink, biasing and connections

Finished circuit with heatsink, biasing and connections

The multiturn trimmer potenciometer sets the operating point of the MOSFET. The drain current without input signal should be around 150…200mA for this type of transistor. Next step is to check the amplification and signal distorsion.

First I connected the oscilloscope to check the voltage gain. Input was connected to the signal generator set to the frequency of interest and signal level 0dBm. The voltage gain measured with the osciulloscope was 20dB or output voltage was 10-times higher than input voltage. The output was connected to the dummy load.

Signal generator with clean stable sine

Signal generator with clean stable sine

The output is inverted (phase between output and input as near 180°):

Output is 10 times higher than input

Output is 10 times higher than input

The supply voltage should be high enough to avoid saturation. This can be checked with the scope. Input is connected to X axis of the scope and the output is connected to the Y axis. The scope must display the line at 45° or very thin ellipse. When the supply voltage is too low, the transistor will saturate.

Proper supply - MOSFET is not saturated

Proper supply – MOSFET is not saturated

Supply is too low - MOSFET is saturated

Supply is too low – MOSFET is saturated

Further it is difficult to estimate the distortion when signals are checked only with scope. Higher harmonics are result of distorted signal (nonlinear operation). The simplest way to measure this is with the spectrum analyser. It is common practice (and the regulation) to keep higher harmonics at least 40dB below signal. The spectrum diagram is also good indicator for proper settings for bias point.

Spectrum diagram with second and third harmonic

Spectrum diagram with second and third harmonic

From the measurement above it can be read the output power of such simple amplifier is 22dBm and harmonics are more than 40dB below.

The QRSS beacon with this simple amplifier can be heard in Europe on 7039.750kHz, CW with 3s “dot”.


Further improvements:

  1. Bifilar windiong for supply coil
  2. Low pass filter

Bifilar transformer instead of simple inductance for supply rail: a “bifilar transformer” is made by taking two equal lengths of wire and twisting them together tightly on the core. I took amidon T5-60 ferrite toroid core and bifilarly wound 12 turns. With such improvement I can now get same output at much lower power supply.

Bifilar transformer for supply and output

New, improved “schematic” with bifilar transformer

And finally, the output low pass filter:

LPF for 7MHz, calculated with Elsie filter design

I took another amidon T5-60 core with 5 turns and two 330pF/NP0 ceramic capacitors.

Final result with 0dBm input signal and 10V supply voltage is:

Gain = 20dB, 2nd and 3rd harmonic is more than 50dB below, Supply voltage = 10V

 

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