# How to make a small amplifier, able to amplify a signal 2 × the input

I have a small device expecting a signal in the 8MHz range of 2V peak-to-peak.

The output of my signal generator (or rather the device that outputs the modulated signal) is only at 1V peak to peak.

My idea is to construct a (very) simple amplifier to boost the signal. The only requirement for this amplifier, is that it is

1. small, simple, and (relatively) low noise
2. non-inverting. Non-inverting is very important, as it is an analog picture signal.

As I understand it, I'd simply need a single transistor, but I'm not sure how to wire it up, so that it acts as signal amplitude doubler or something. The signal source is stable, i.e. the amplitude never goes beyond 1V and the amplitude is never lower.

Perhaps using an op-amp circuit would be better, given the output is not necessarily high impedance?

I'm not sure where to start, even though the application is "pretty simple" at first glance.

• The proper solution needs a design spec for output impedance and load current in order to define the load error from RLC impedance @ 8MHz May 12, 2020 at 3:49
• Single supply rail-to rail op-amp. Or maybe a video buffer can be set up with a gain of 2. May 12, 2020 at 4:13
• 8 MHz is fast enough that not just any op-amp will do. Trying to do it with transistors will be pretty challenging, so I do recommend finding the right op-amp. May 12, 2020 at 7:21
• What's the lowest frequency of interest to be amplified? May 12, 2020 at 8:10
• @mkeith around 50Hz, as this is technically the field frequency, etc. May 12, 2020 at 13:28

Edit: added suitable op amps for application and edited some information to be more accurate

For your specific application, an op amp circuit would work well. I’d recommend searching on DigiKey.com or another electronic supplier for op amp that has plenty of gain bandwidth, such as 200MHz or higher, low noise, and small package. For example, the OPAx354 has bandwidth of 250MHz, is rail to rail, single supply operation, and low noise. Another option is the ADA4891 which when designed at gain of +2, can be supplied with 5V and have 120MHz of -3db small signal bandwidth. It has a nice application design in the Datasheet. As an extra, LT1815 is also suitable.

As for the design, you’d need to decide if you want to have single supply or dual supply operation. Since you want gain of 2 and non-inverting then you’ll go with non-inverting op amp configuration. To minimize noise generated in resistors, don’t use very large resistors such as in the megohm region.

For non inverting op amp, gain is G=R2/R1+1. To obtain gain of 2, you might choose resistors R1=R2=1k (1%) or 499ohm (1% ) or 604 ohm (1%) But use standard 5% tolerance resistors or better. For higher precision, use 1%.

Shown below is general configuration of non inverting op amp( supply rails and single supply bias not shown).

To answer your question in comment. If you’re signal stays positive, you can use single supply non inverting op amp design. Note: you’ll need to adjust this design example to your needs.

• Assuming the signal never goes negative, I'm assuming I also don't need a negative power supply, instead I just need 'V+' to go to +5V and 'V-' to GND? May 12, 2020 at 17:49
• If your signal never goes negative then you can use just single supply and ground. You’ll have to configure your amplifier as single supply non inverting. You’ll have to adjust the values but check this source for example of design analog.com/en/analog-dialogue/articles/… May 13, 2020 at 15:50
• Thanks, the links are really nice to have. May 16, 2020 at 17:00

You can use a good video op-amp to do this. Eg. AD8047/8 (GBW = 160MHz), provided your output load is 75$$\\Omega\$$ or greater.

The AD8047 is shown, which is stable at a gain of 1, so it's less likely to break into oscillation with a gain of 2. You'll note that the phase difference between input and output at 8MHz is fairly small. The AD8048 would be better in that respect.