RF power amplifiers
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Low power VHF RF amplifier with VMP1 Mosfet
(IZ1BTS Joe & IK1XPD Max)
We describe the realisation and test of a five watt linear amplifier for the 2 meters band
with a VMP1 Mospower FET power transistor from Siliconix (now a brand of Vishay Corp.).
The idea comes from an article "Two-meter transverter using power fets" published on Ham Radio
September 1976 (pp. 10-15) by WB6BPI, Larry Leighton.
The desirable features of Mos power transistor are
Unfortunately Siliconix had discontinued the production of this and similar RF components
(VMP-4, Si-8910 Mos quad for DBM) nevertheless we hope this note may be a source of ideas
for your next RF amplifier with similar power Mosfet devices like DV-1210, DV-2810....
- no thermal runaway;
- no secondary breakdown;
- relatively high input and output impedances with respect to bipolar transistor;
- low sensitivity to load mismatch.
The electrical circuit (from Ham Radio Sept. '76)
VMP1 was manufactured in a TO-3 case with drain connected to the case.
To heatsink the transistor without adding too much output capacitance I used (like in the original paper) a beryllium-oxide heatsink insulator 1.6 mm thick.
WARNING: precautions must be used working with beryllium-oxide insulators.
Pulverized particles are poisonous if breathed.
If you select a different kind of insulator you must change the matching networks values.
VMP1 amplifier performances
The amplifier is able to deliver 5 Watt (+37 dBm). The -1 dB compression point is at +39 dBm.
The power supply voltage was +28 V. The quiescient drain current was adjusted to 200 mA; higher drain bias current gives no improvements on IMD test (next).
In this figure you can see the input return loss R.L. or S11 (curve number 1)
and the gain S21 (curve number 2) measured with an RF network analyzer.
The gain is about 12 dB and the R.L. is lower than 30 dB in the 2m band.
Two-tone, third order intermodulation measurement set-up.
The spectrum plots here reported shown the input and output signals from VMP1 amplifier.
The Intercept point (IP), with reference to a single-tone output, is IP(dBm)=Pout(dBm)+Pimd(dB)/2 = 47 dBm
The spectrum of the input signal on the VMP1 amplifier (IMD_out) is not perfect. It shows a small third order intermodulation contribution from the previous
stage (a CATV amplifier module), 60 dB lower than single tone level. This undesired contribution gives a negligible effect on the measured Intercept Point (IP) level.
from p.231 of Introduction to Radio Frequency Design, Wes Hayward, W7ZOI, ARRL: "Some large
signal VMOS-FETs offer high output intercepts. However, they often require very high bias
power to achieve this, perhaps Vdd=25 V and Id= 400 mA. Examination of the
low frequency characteristics are initially puzzling, for a curve of Id versus
Vgs is extremely linear, lacking in both second and third order curvature.
However, the parasitic capacitances are high. Moreover, they are strongly dependent
upon the drain voltage, perhaps accounting for the poor intercept efficiencies."
The amplifier noise figure has been measured with HP8970 and HP346A head without trying to reduce it.
Here the noise figure values versus frequency.
|Frequency [MHz]|| ||Noise Figure [dB]|
- Larry Leighton,WB6BPI, Two-meter transverter using power fets, Ham Radio, September 1976, pp. 10-15,
see also VHF/UHF Manual, 4th Ed., G.R. Jessop (G6JP), Power-fet transverter for 144 MHz, pp. 6.16-6.18;
the schematic here reported refer to a Siliconix VMP4 but the photos (taken from the original paper by WB6BPI) are relative to a VMP1 device.
- Ed Oxner, ex W9PRZ, Mospower fet as a broadband amplifier, Ham Radio, December 1976, pp. 32-35.
- Ed Oxner, ex W9PRZ, Build a Broadband ultralinear VMOS Amplifier, QST, May 1979, pp.23-26.
- Ed Oxner, ex W9PRZ, VMOS Power FETs in your next Broad band Driver, Technical Article TA76-1, Siliconix, May 1978
- Ed Oxner, ex W9PRZ, A New Technology: Application of VMOS Power FETs for High-Frequency Communications, Technical Article TA76-2, Siliconix, May 1978
- H. Braubach, DL1GBH, V-MOS Transistors in Power Amplifiers for 144 MHz, VHF Communications 3/1983, pp.130-140
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