• RF-Amplifier-3W-700MHz

Vorläufig / Preliminary, work in progress

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RF Amplifier 3W

In Chinese shops you can buy for about 12 EUR an RF-Amplifier module with an advertised output power of 3 W.

On the right is a picture of the module.

Let's see with some measurements, if it does so.

Specifications

1. Supply voltage:DC+12~15 V (typical 15 V 0.3-0.5 A)
2. Working frequency: 2 MHZ-700 MHZ, input and output impedance: 50 ohms;
3. Maximum output power: 34.8 dBm (3W)

4. Maximum input power: <+10 dBm

5. Typical power Gain: 35 dB (> 300 MHz Gain slightly decreased)

6. Size: 7 x 3 cm / 2.76 x 1.18 in
7. Weight: 28 g

Measurements

I use the following equipment:

1. Adjustable Power Supply with current limiting.

2. Step Attenuator 0 - 31 dB

3. Attenuator 20 dB (SMA connectors)

4. RF Wattmeter RW-151D, company Kuranichi Keisokui (5, 25, 150 W fullscale)

5. RF Testgenerator frequency 10 MHz, output power +4 dBm at 50 Ohm, waveform: sinus.

6. DSO Hantek DSO5072P

7. Coax cables to connect

Result

• The input power was varied between -16 dBm to -22 dBm

• The gain factor was about 46.6 - 49.6 dB

• At 12 V supply the maximum output power (sine) was 0.7 W = 28.6 dBm = 17.0 Vpp

• At 15 V supply the maximum output power (sine) was 1.2 W = 30.7 dBm = 21.7 Vpp

• The current draw was 0.54 A to 0.58 A

• The power efficiency was 11% to 15%

See following the DSO screen shots from the output signal:

The left screen shot shows an overdrive at Vcc = 12 V, on the right you see the output with the same drive level but at Vcc = 15 V.

If you want to see more details of the measurement, have a look at this spread sheet (Open Office).

Frequency response

With the Vector Network Analyzer nanoVNA and some attenuators it was possible to measure the frequency response.

See below the setup:

.

See below the the frequency response measurement:

.

There is no gain linearity specified. When you look at the gain specification, it says 35 dB. Then it is +13 / -5 dB, what is OK. The sentence (> 300 MHz Gain slightly decreased) means in reality about 5 dB.

Cooling

While testing the amplifier, I found that the heat sink gets pretty hot. So, I limited my measuring time to less than a minute. With a lower quiescence current of 0.3 A, see chapter Improvement, I measured with an infrared thermometer the package temperature and calculated a thermal resistance of the mounted heat sink to about 8.4°C/W.

Some details for calculation:

• MOSFET U2 label: K2 1Y1, found no data sheet
• Heat sink: 30 x 70 x 12 mm, area = 21 cm2, estimated thermal resistance 8.4°C/W
• U2 Package: SOT-223, thermal resistance 15°C/W
• Operating Junction Temperature, Limit 150°C
• Operating Junction Temperature, safe Limit 130°C
• Maximum safe temperature rise at room temperature of 30°C = 100°C

• Thermal resistance of thermal compound: 1°C/W

• Total thermal resistance: 8.4 + 1 + 15 = 24.4°C/W

• Maximum safe power loss: 100 / 24.4 = 4.1 W

With the previous setups (0.5 A quiescence current) it is no possible to get a 100% duty cycle operation, without additional cooling.

For more background info see PA Cooling.

Improvement

What wonders me was the high quiescence current. So, I connected a trim pot of 10 KOhm parallel to R6 and adjusted the quiescence current for best efficiency at an output sine waveform at 10 MHz. That gave a resistance of 5.6 KOhm in parallel to R6, and a quiescence current of about 300 mA. Now the values are:

• The input power was varied between -18 dBm to -20 dBm for output sine wave

• The gain factor was about 48 dB

• At 12 V supply the maximum output power (sine) was 0.7 W = 28.6 dBm = 17 Vpp

• At 15 V supply the maximum output power (sine) was 1.1 W = 30.4 dBm = 21 Vpp

• The current draw was 0.3 A

• The power efficiency was 19% (12 V) to 24% (15 V)

• The power loss was 2.9 W (12 V) or 3.4 W (15 V), allows 100% duty cycle (max 4.1 W)

Summary

Within the measured values this amplifier is useful, especially considering the price. You just have to consider, that it is not a broad band linear amplifier. If you do not want to mount a fan to the heat sink if you need 100% duty cycle, apply my Improvement.

Links

1. www.banggood.com search for amplifier

2. dBm-Watt-Voltage table

3. dBm Voltage calculator

4. dBm to Watt calculator

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-- RudolfReuter 2020-05-17 17:50:40

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