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Dual RF Amplifier for Signal generators This small RF amplifier has two outputs with individually selectable gains. This makes it suitable to add to a signal generator to provide a higher output level, or for better drive strength, or ‘fanning it out’ to multiple other pieces of equipment and more. by Charles Kosina any signal generators do not M provide a high enough output level for certain uses. This small PCB uses an OPA2677 high speed dual op-amp to boost signals of 100kHz75MHz at around 0dBm (1mW, 225mV/-13dBV into 50W) to around 18dBm (63mW, 1.78V/5dBV into 50W). The OPA2677 has impressive specifications. It can operate on voltages from 3.3V to 12V, has rail to rail outputs, a high drive capability and a gain bandwidth (GBW) of 200MHz. But what makes it stand out is a slew rate of 1800V/µs, which means it can provide a large output swing for high-­ frequency signals. Because it is a dual op amp, my design provides two outputs for the one input signal. Individual feedback resistors and a potentiometer set the gain for each output. The maximum gain is 1 + (470W ÷ 68W) = 7.9 times with the 1kW single-­ turn trimpot set to minimum. The lowest gain is 1 + (470W ÷ 1068W) = 1.44 times with the trimpot set to maximum. The output impedance is 50W and it will safely drive a 50W load. The power supply voltage should ideally be in the range of 9-12V. You could use 5V DC, but the amplified signals will be limited to 5V peak-to-peak at the op amp output and 2.5V peak-topeak at the 50W load, or 884mV RMS (13.9dBm/24mW). The maximum output with a 12V supply is about 25dBm, as shown in the specifications panel. The Amplifier is useful from 100kHz to 75MHz, although once past 50MHz, the maximum output level starts to drop off. Table 1 shows spot measurements at several frequencies using my signal generator as an input. The output variability somewhat depends on the signal generator variation in output level. The OPA2677 is not cheap, about $9 from Digi-Key, Mouser or element14, but I bought five from AliExpress for $14.50. Still, even if you pay $9, the overall cost of building this Amplifier Features and Specifications ∎ Operating frequency range: 100kHz to 75MHz ∎ Number of inputs: 1 ∎ Number of outputs: 2, individually gain adjustable ∎ Gain range: 1.44 times (3dB) to 7.9 times (18dB) ∎ Maximum output level: 25.6dBm <at> 30MHz (360mW into 50Ω, 12.5dBV, 4.25V RMS) 23.2dBm <at> 50MHz (207mW into 50Ω, 10dBV, 3.2V RMS) 13.5dBm <at> 70MHz (22mW into 50Ω, 0.51dBV, 1.06V RMS) ∎ Power supply: 9-12V DC <at> 20-25mA (or 5V DC with reduced maximum output levels) 32 Silicon Chip Australia's electronics magazine is modest. See the panel at the end of the article on the short-form kit. Circuit description The whole circuit is shown in Fig.1. The signal fed in via SMA connector CON1 is AC-coupled to both halves of dual op amp IC1 via 100nF capacitors. These signals are biased to half the VCC rail (eg, 2.5V for a 5V supply or 6V for a 12V supply) using 470W resistors. Those coupling capacitors and bias resistors form high-pass filters with a corner frequency of 3.4kHz (1 ÷ [2π × 100nF × 470W]) so they will not attenuate signals within the specified operating frequency range, from 100kHz to 75MHz. The signals are coupled to the non-inverting input pins, so the amplifiers do not invert the signal phase. The outputs of the op amps (pins 1 & 7) are fed back to the inverting inputs (pins 2 & 6) via 470W resistors, which form voltage dividers with trimpots VR1/ VR2 and their series 68W resistors. The 100nF capacitors in the feedback network reduce the DC gain of these amplifiers to 1x so that the input offset voltages (up to 5.3mV) are not amplified. The corner frequency of the high-pass filter formed is similar to that of the input networks as the component values are the same. As mentioned earlier, the op amps have very high gain bandwidths (GBW) and slew rates, so they are effective up to high frequencies. Because the gain bandwidth is fixed, the maximum signal frequency drops as you increase the gain. For example, with siliconchip.com.au the GBW of 200MHz, a gain of four times is possible at 50MHz or about three times at 70MHz. The outputs of the two op amps are coupled to SMA connectors via 100nF capacitors to eliminate the VCC/2 DC bias and fed through 51W resistors for impedance matching. You could change them to 75W if you need to feed into a 75W device. The VCC/2 rail is formed by a simple 1.2kW/1.2kW voltage divider with a 100nF capacitor from the junction to ground to eliminate supply ripple and keep the source impedance low at higher frequencies. Op amp IC1 also has the obligatory 100nF supply bypass capacitor. Note that there is no termination resistor for input CON1. You could add an M2012/0805 size resistor (51W or 75W) across the terminals of the SMA socket if you need one. Construction Construction is relatively straightforward as there are only a couple dozen components total. The Dual RF Amp is built on a double-sided PCB coded CSE220602A that measures 38 × 38mm. Refer to the PCB overlay diagrams, Figs.2 & 3, to guide you during assembly. Start by fitting the SMDs to the component side, with IC1 first. Determine its pin 1 location – look for a dot or divot in one corner, or failing that, a chamfered edge on the pin 1 side. Table 1 – frequency vs maximum output level <at> 12V DC Frequency Output (p-p) Output (RMS) Output (dBm) Output (dBV) 1MHz 9.5V 3.36V 23.5 10.5 10MHz 8.4V 2.97V 22.5 9.5 20MHz 10.0V 3.54V 24.0 11.0 30MHz 12.0V 4.24V 25.6 12.5 40MHz 9.6V 3.39V 23.6 10.6 50MHz 9.1V 3.22V 23.2 10.2 60MHz 5.6V 1.98V 18.9 5.9 70MHz 3.0V 1.06V 13.5 0.51 Locate it with pin 1 towards the upper right with the PCB orientated as shown in Fig.2. Add flux paste to its pads, then tack one pin with a bit of solder and check the alignment of the other pins. If they are good, solder the diagonally opposite pin. Otherwise, heat the original solder joint and gently nudge the part until it is in place. Then solder the remaining pins, refresh the first one and clean up any solder bridges which might have formed between pins with another dab of flux paste and some solder wick. Clean flux residue off the board with alcohol or a flux cleaner and inspect the solder joints to ensure they are all good. Then proceed to fit the passives, none of which are polarised, using a similar technique of tacking one side, then adjusting the alignment and after a brief delay to allow the solder to solidify, solder the other side. Fig.1: the Dual RF Amp is a straightforward implementation of the OPA2677 dual high-bandwidth op amp. Signals are AC-coupled at the inputs and outputs so they can be DC-biased to a half supply rail formed by two resistors and a capacitor. Trimpots VR1 & VR2 adjust the feedback ratio and thus the gain of each individual amplifier. siliconchip.com.au Australia's electronics magazine The resistors will be marked with codes indicating their values (eg, 122 or 1201 for 1.2kW), while the capacitors will not be marked, but they are all the same value (100nF). When all the SMDs are mounted on that side, flip the board over and solder the lone capacitor on the other side. That just leaves the six through-hole components: two trimpots, the power header and the three SMA sockets. It’s best to fit the SMA sockets next, so you have good access to their pins. Push them down fully and solder all five pins, keeping in mind that you may need some extra heat or flux to solder the four outer pins due to their thermal mass. Finally, mount the two trimpots and the power header. Use single-turn trimpots as multi-turn types likely have too much inductance. You could Figs.2 & 3: most components are SMDs that mount on the rear, while one capacitor and the three SMA connectors are on the front. The RF connector side of the board is covered with a ground plane. May 2023  33 solder some figure-8 wire directly to the board for power, but a polarised header is more convenient. Its exact orientation doesn’t matter as long as you observe the + and – markings when wiring it up. Housing it As the board is small, it can fit into most cases. A metal case is preferred for RF shielding. See the parts list for suggestions and note that the 51 × 51mm diecast cases sold by Jaycar and Altronics are too small to fit the PCB. Fig.4 shows the hole positions to drill in the lid or base, and the board can then be mounted using the SMA connector nuts. Drill a hole in the side of the case to fit a chassis-mount barrel socket and wire it up to CON4. Double-check that the positive wire (usually the tip of the barrel socket) goes to the + side of CON4, as the board has no reverse polarity protection. There isn’t a great need for a power switch as you can simply unplug the plugpack from the wall when you aren’t using it. Still, if you want to add a power switch, all you have to do is drill a hole in a convenient location, mount the power switch and wire it in series with the positive conductor from the barrel socket to CON4. If you want to add reverse polarity protection, solder a 1N5819 diode to the barrel socket with its anode to the positive tab of the socket, then run the supply wire to the board or switch from its cathode. That will drop the supply voltage slightly, by around Parts List – Dual RF Amplifier 1 double-sided PCB coded CSE220602A, 38 × 38mm 1 diecast aluminium case, large enough to fit the PCB [eg, Jaycar HB5062, 111 × 60 × 30mm] 1 9-12V DC 50mA+ plugpack or other DC supply 1 OPA2677IDDA dual high-bandwidth op amp, SOIC-8 [element14, Mouser, Digi-Key] 2 1kW single-turn 3362P-style top adjust trimpots (VR1, VR2) 8 100nF 50V X7R SMD ceramic capacitors, M2012/0805 size 3 vertical SMA female sockets (CON1-CON3) 1 2-pin polarised header with matching plug and pins (CON4) 1 chassis-mount DC socket to suit plugpack plug 1 short length of light-duty figure-8 cable 1 chassis-mounting SPDT switch (optional; power switch) 1 1N5819 schottky diode (optional; see text) Resistors 2 1.2kW 4 470W 2 68W 2 51W 0.3V, so it may have a small impact on the maximum output signal level. Finally, you might want to drill a couple of small holes in the face of the case opposite the board so that you can slot in a thin adjustment tool to adjust trimpots VR1 and VR2 with the case closed. That depends on your application; you could just set a different fixed gain for both trimpots and then use whichever output suits your needs at the time. Before screwing on the lid, unplug the CON4 plug from the board, connect your power supply to the barrel socket and use a DMM to check that the power polarity at the plug is correct. Then plug it in and connect a signal to the input socket. Verify that an amplified version of the signals appears at the outputs using a scope, signal level meter or frequency counter, depending on what you have on hand. Using it There isn’t much to it – just power it up, feed in your signal, adjust the level using trimpot VR1 or VR2 if necessary, and take the output from the corresponding socket. The CON2 signal level/gain is adjusted using VR1, and the CON3 signal level/gain is adjusted using VR2. Keep in mind that VR1 and VR2 are wired such that anti-clockwise rotation increases the gain and clockwise rotation decreases it. This article is in memory of Rod Graham, VK3BQJ, who passed away SC on November 4th 2022. SC6592 Kit ($25 + P&P) Includes the PCB and all onboard parts. You only need to add a case, DC socket, wiring and the optional power switch. Fig.4: just about any metal case would be suitable but this one is relatively compact. The lid is larger than the base, so if using this as a template, cut it to the appropriate outline. The central area could be cut out and transferred to just about any other case. The hole in the side for the power socket is not shown here; it could go just about anywhere. 34 Silicon Chip Australia's electronics magazine Compared to the lead image, which is shown enlarged, here is the finished Dual RF Amplifier shown at life size. siliconchip.com.au