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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.

attachment:RF_PA_3W_top_IMG_2199.JPG

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

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.

attachment:RF_Power_Amplifier_3W_2-700MHz_12V.png

attachment:RF_Power_Amplifier_3W_2-700MHz_15V.png

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:

attachment:nanoVNA_Amplifier-3W-700MHz_IMG_2200.jpg

.

See below the the frequency response measurement:

attachment:nanoVNA2_RF-Amplifier_3W_700MHz_12V.png

.

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:

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:

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.

  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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RF-Amplifier-3W-700MHz (last edited 2020-05-20 14:49:16 by RudolfReuter)