Mods and improvements to the SPF5189Z LNA board

The SPF5189Z is a high-performance pseudomorphic high-electron-mobility transistor (pHEMT) monolithic microwave integrated circuit (MMIC) low noise amplifier (LNA) designed for operation from 50MHz to 4000MHz. Since the chip has a small SMD capsule and to avoid damage from improper soldering tools I bought a couple of boards based on SPF5189Z. With its large bandwidth, this LNA can be used to improve the reception of my RTL-SDR. Usage is not limited to SDR, this amplifier can boost FM radio or TV signals. TO get the best results, I had to mount it as close as possible to the antenna. By studying the datasheet and looking at the boards I have bought, I realized that design wasn't quite made for what I wanted to achieve.

First of all, power is supplied separately, not over coaxial cable. To fix this, I would have to build a bias tee circuit. But, before doing this, I took off the metal cover off only to find that to power SPF5189Z, they used a bias tee circuit. So, I would have to add another one just to redirect DC back to the RF line. Secondly, the RF input is capacitor coupled. Just that. No ESD protection. I'll have to do something with this too.

The SPF5189Z LNA board

Stereo tone controller with transistors

The tone controller is a circuit inserted before an audio power amplifier. Its purpose is to allow the user to adjust the gain of specific frequencies of the audio spectrum. Tone controllers range from simple bass boost circuits to complex equalizers. The circuit presented here is a simple one, with a potentiometer which alters the lower third of the audio spectrum (for bass adjustment) and another one which alters the upper third of the audio spectrum (for treble adjustment).

The schematic does not contain a volume adjustment potentiometer, but that can be added before the controller input. The signal input level should not exceed 1 Vp-p. When both potentiometers are turned up to the middle (flat), the relative gain of the circuit is -1 to -2 dB. Therefore, with an input signal of 1 Vp-p, you get no less than 0.8 Vp-p at the output.

Built prototype of the stereo tone controller

Stereo audio amplifier with TDA2003

TDA2003 is an integrated audio amplifier circuit capable of providing up to 10 W into 2 ohms load and 6 W into 4 ohms load when powered at 14.4 V. It is very easy to build a reliable circuit with it because it has short circuit protection. It will withstand a permanent short circuit on the output as long as supply voltage doesn't exceed 16 V. The maximum operating DC voltage is 18 V, however TDA2003 will not get damaged as long as supply voltage is less than 28 V. It comes with integrated thermal limiting circuit.

Having these features, the TDA2003 proves to be a good option for small power amplifiers. It was designed for car audio, that's why it is powered from single supply of about 12 V. Although nowadays it is considered obsolete, there are plenty of electronic parts suppliers which have TDA2003 in stock. At very low prices, by the way. Using the datasheet as source of inspiration, I designed my own PCB for two TDA2003 circuits, to make a stereo amplifier.

The TDA2003 amplifier on homemade PCB without heatsink

HF & VHF antenna amplifier without coils

The following receiving amplifier can be used for any kind of signal in the HF and VHF bands from about 1 MHz to 400 MHz. It offers quite linear gain across this band width without using any LC tuned circuits. Therefore it can be used for shortwave, FM and VHF signals and even AM band can be received, because the gain at 500 kHz is not less than 18dB. The circuit does not use coils. However, a choke will be required if you will power the amplifier through the feed cable. The circuit uses five RF transistors with a cut-off frequency of at least 600 MHz. The PCB is designed for the (S)S9018 transistors (EBC pinout), but you can use any kind of low power RF NPN transistors, like MPSH10 and even the old types like BF200 or BF214 (pay attention to pinout). S9018 transistors are cheap and widely available (you can find them in transistor packs).

The amplifier's input is unbalanced and it can be connected to any kind of antenna (after the balun if it is necessary). Because Q5 is placed in a common base configuration, the input impedance is high, thus the amplifier can be used with whip antennas too (they usually have high impedance at low frequencies). The output is 75 ohms unbalanced. The overall gain of this amplifier is 20 dB.

10W Stereo Audio Amplifier with Transistors

Most audio amplifiers nowadays are built with special integrated circuits. These ICs are getting smaller, but the power they can deliver and efficiency are increasing. The following amplifier is a classic design built with common parts and some power transistors in output stages. It can deliver a maximum of 10 W into 4 ohms speakers on each channel when it is fed with a 0.5 V peak-to-peak signal. Input impedance is above 100 kilo-ohms. The amplifier should be powered from 24 V regulated supply.

The output transistors must be able to handle a collector current of at least 2 A and dissipate at least 20 W. Something like BD237 & BD238 or BD 437 & BD 438 pairs will do. The rest are general purpose transistors such as BC547, BC171, 2N2222, S8050, 2N3904 and their complementary BC557, BC177, 2N2907, S8550, 2N3906 (pay attention to pin order). The drivers of the power transistors, just like them, must be complementary and with similar current gain. So, if you have a hFE meter, it's recommended to test the transistors and match them based on hFE. If you can't measure them, follow the hFE markings. Do not match a BC547B with a BC557C because they have different gains.

Built channel amplifier (without heatsink)

2.5W Audio Amplifier with Transistors

This amplifier is easy to build using common discrete parts. A channel uses only four general purpose transistors (3 NPN like BC547, 2N2222, 2N3904 and 1 PNP like BC557, 2N2907, 2N3906) and two medium power complementary transistors like BD135/137/139 with BD 136/138/140 or MJE200 with MJE210. Almost any pair of complementary transistors will do as long as hFE is greater than 60, maximum collector current at least 1 A and dissipation at least 10 W.

The power transistors must be fitted on a heatsink! They have their collectors electrically connected so you don't have to isolate each other. The heatsink should have an area of at least 40 square centimeters. Note that the heatsink voltage is neither ground or supply voltage so do not connect it with anything else besides transistors. Be careful not to connect it with chassis ground. If you build the other channel for stereo use, do not electrically connect the heatsinks of both left and right channels.