Could a LMH6654 op amp work as a HF-Band reciever? - Page 1
Forum section: electronics / rf-microwave

author: mildTraumaticBrainInjury
author_rank: Newbie
timestamp: September 20, 2022, 07:53:01 pm
content: Hi All,Apologies, I have no formal electronics training, so this may be well off-the-mark.I was wondering if it was possible to use a standard LMH6654 op amp (https://www.ti.com/lit/ds/symlink/lmh6654.pdf) as an amplifier for a lower-half of HF-band reciever. While in theory it has 20dB of gain below 10MHz, and a datasheet given typical value for harmonic distortion and voltage noise, it doesn't list anything RF-related in it's example applications section. When looking through digikey, most of the cheap amplifiers in the "Instrumentation, OP Amps, Buffer Amps" section didn't seem to list any example RF applications.I'm of course not expecting any cellular applications for hundreds of megahertz of gain bandwidth product amplifiers, but what it is that makes these op amps suitable for e.g. video applications but not lower frequency RF applications? After all the amount of voltage gain and the frequency response seem enough for this purpose.Thanks, mTBI

author: jwet
author_rank: Frequent Contributor
timestamp: September 20, 2022, 11:12:03 pm
content: This is the kind of question that experienced engineers would really scratch their head over- mainly why? but as a beginner, I'm sure you have your reasons.The best short answer is no, the LMH6654 isn't really a suitable device to make a good low band HF receiver. It could be forced to if you were on a desert island or in a design contest that required you to use the LMH6544 If you read on, I try to tell you why.1. First- a receiver needs a ton of gain- you're taking at a minimum a few microvolt signal and getting it up to say a volt for the audio amp- this is a gain of 1 million- 120 dB. This is just to get you calibrated- this isn't the main problem. The signals also aren't always a microvolt, they can have a tremendous dynamic range, up to millivolts. This dynamic range problem is one of the real challenges of a good reciever. Imagine listening for a tiny signal with a booming signal a few kHz away.2. Receivers are more than just amplifiers though it's generally one of their embedded functions. They need to be VERY frequency selective and pretty low noise to operate at all. The LMH6544 might be ok for noise actually. This selectivity is actually part of the noise solution- you need to reduce the bandwidth in order to not have the noise just swamp everything. They also require some form of demodulation and generally automatic gain control to handle a wide range of signal input levels. Imagine hooking an antenna to a 100 Mhz oscilloscope- without selectivity, all you'd be doing is looking at all noise in the world from DC to 100 MHz.3. The LMH6654 could be used as a gain stage in an RF or IF amplifier but its not particularly well suited to this and is somewhat expensive. A few good transistors would likely do a better job and would cost a quarter perhaps. Op-Amps are sometimes used in RF applications but its not common. There are generally better ways to do it. Op-Amps excel at generally being very linear, broadband and low distortion. In RF circuits, by using narrowband techniques, which is also a necessity, these attributes don't really fit.4. The LMH6654 has its bandwidth deliberately cut back considerably to make it unity gain stable- in a radio you generally want lots of gain spread over multiple tuned stages for stability and control of feedback/oscillation. A device that is not deliberately limited in BW would be better- look at the OPA656 and OPA657 for an example of a pair of amps, the 656 is unity gain stable, the 657 is only stable for gains of 7 more or more. You can see the benefit of a non compensated op-amp- the BW is considerably better.An old op-amp like device that was pressed into service in receivers (by hams mostly) at times was the LM733. It has three fixed gain values built in, is diff in and diff out- suitable for IF transformer type drive and is low noise. Discrete transistors in nice configurations like cascode or common base in narrow band tuned circuits are very effective for RF.I hope this helps. You might to look at a decent book on receiver design or build an AM radio kit. The ARRL Handbook is the standard Ham's reference and is pretty approachable. Any public library would likely have one or something like it.Regards,John

author: M0HZH
author_rank: Regular Contributor
timestamp: September 25, 2022, 03:40:54 pm
content: It really depends where you intend to use it in the receiver chain; it can be used successfully but generally there are better, RF dedicated parts, with lower noise, better IMD performance or well documented applications.It can still perform quite well as the first amplifying stage after a Tayloe / H-Mode mixer for example, in a zero-IF SDR receiver.

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 25, 2022, 04:40:35 pm
content: You "could" use it but it is not appropriate for the way receivers with IF stages are designed, stages with fixed IF frequencies and bandwidthsBy the way the LMH6624 would be a better choice, more gain and more bw.

author: David Hess
author_rank: Super Contributor
timestamp: September 25, 2022, 08:45:15 pm
content: The differential input topology doubles input noise versus using a single transistor while requiring more power, so operational amplifiers are not suitable where the lowest noise is desired, like an RF input stage. Differential amplifiers are common in integrated receiver chains, but only where added noise is not a concern. Check some old Fairchild parts, some of which were just a few transistors on one IC.The frequency compensation lowers the possible performance significantly.Basically you get better performance for less cost with just a few transistors, or even just one.