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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
M
any signal generators do
not 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.
Since 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 RF 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 £5
from Digi-Key, Mouser or element14,
but I bought five from AliExpress for
£8. Still, even if you pay £5 (each), the
overall cost of building this RF Amplifier is modest.
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 ×
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)
26
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 and 7) are fed
back to the inverting inputs (pins 2 and
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 highpass 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 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
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 a
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.
Practical Electronics | May | 2024
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
Construction
Construction is straightforward as there
are only a couple dozen components.
The Dual RF Amplifier is built on a double-sided PCB coded CSE220602A that
measures 38 × 38mm, which is available
from the PE PCB Service. Refer to the
PCB overlay diagrams, Fig.2 and Fig.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. Locate it with pin
1 towards the upper right with the PCB
oriented as shown in Fig.2.
Add flux paste to its pads, then tack
one pin with 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.
Next, fit the passives (none 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.
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 solder
some figure-8 wire directly to the board
for power, but a polarised header is more
Compared to the lead image, which
is enlarged, here is the finished Dual
RF Amplifier shown at life size.
convenient. Its exact orientation doesn’t
matter as long as you observe the ‘+’ and
‘–’ markings when wiring it up.
Housing it
This board is small, so 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,
Dual RF
Amplifier
Fig.1: the Dual RF Amplifier 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 and VR2 adjust the feedback ratio and
thus the gain of each individual amplifier.
Practical Electronics | May | 2024
Figs.2 and 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.
27
Parts List – Dual RF Amplifier
www.poscope.com/epe
- USB
- Ethernet
- Web server
- Modbus
- CNC (Mach3/4)
- IO
- PWM
- Encoders
- LCD
- Analog inputs
- Compact PLC
- up to 256
- up to 32
microsteps
microsteps
- 50 V / 6 A
- 30 V / 2.5 A
- USB configuration
- Isolated
1 double-sided PCB coded CSE220602A, 38 × 38mm from the PE PCB Service
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
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 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.
Reproduced by arrangement with
SILICON CHIP magazine 2024.
www.siliconchip.com.au
This article is in memory of Rod
Graham, VK3BQJ, who passed away
on 4 November 2022.
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.
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28
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.
Practical Electronics | May | 2024
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