Design Literature for the analog gang - 2016/17

Why use a rail splitter?

The ground point in a circuit should be able to BOTH source AND sink currents. The V/2 potential provided by an R+R resistor divider across a V-0 potential is simply that: its a potential. It is able to source current from +V into the circuit through the upper R. But can't sink any. Think about the impedance looking back into the R+R divider from the load circuits' perspective. Usually R+R must be kept very large to minimize idle current consumption.

A real 'ACTIVE' ground is needed for a dual supply opamp. Here is an article that gives you TEN best ways to do it.

PCB design fundaes

Here's an excellent hands-on how-to guide: Tim Williams' Circuit Designer's Companion There are also a couple of other books on the Secrets of good RF analog design by Behzad Razavi in the Central Library

Gain peaking

What causes it? How to fix it

Sometime you get a slight peak near the first corner frequency. This is called Gain peaking. It will cause your circuit to spontaneously break out into oscillations at the corner frequency. It is almost always some capacitance that is the culprit in opamp feedback circuit. The sharper the peaking, the higher the Q factor associated with such resonant oscillations.

Here are some tips on how to do stability analysis and isolate such problems:

• Some lecture notes from UCSB: See page 12
• Texas Inst tech note on how stray capacitance can cause such unwanted gain peaking even in single pole feedback circuits
• Here's a tip from the clever engineers at Texas Inst: Tech Note # 185 on how to use active phase correction to fix such gain peaking. Note that this increases the circuit complexity significantly! Useful as a thought experiment - often unnecessary unless you are getting into the REALLY high performance game.
• This OCW lecture explains Q factor in a much simpler passive resonant circuit
• You can also think about it in a different context altogether: How can you swing in a swing?? when every force has an equal and opposite reaction.

Powering your circuits

• TLE246 is the swiss army knife to create virtual grounds i.e. split rail supply from a battery/unipolar supply
  Here are some links on usage:
  • http://www.ti.com/lit/ds/symlink/tle2426.pdf
  • What are the different types of virtual grounds? See this note

Attenuation

• Feedback circuits with gain less than 1 are usually tricky. Here are a couple of tech notes that point out why:
  • From Texas Instr : Tech Note Engr to Engr
  • Tim Green venerated desiger also rom Texas Instr : Tech Note 6/15
• In practice it usually comes down to impedance matching 50 ohms in-out. Mini-circuits makes the best ones: the attenuating transformer (yes it is usually a hand-made passive!) is built into the sma connectors :
  Often the passives are transformers built into a tiny silicon IC (as we discussed): Mini-circuits makes a range of them with 2.5GHz bandwidth and many attenuation levels. Catolog link. Board layout of how to put such a device between two SMA connectors so that it is 50 ohm transparent is here - Note the trace length/widths!
• Shortwave radio hobbyists often roll their own also :
  

Grand objective

Build a TRANSCONDUCTANCE amplifier. Note that this is the INVERSE of a transimpedance amplifier. Texas Instruments makes a few such IC's : Product List

• OPA860D (74 MHz)
• OPA861D (370 MHz)
• OPA615D (730 MHz)

Other fundaes

Generally we use FR4 PCB substrate material (because it's cheap!) However, for high frequency applications, dielectric constant of the substrate becomes a very important parameter. A different hf-specific substrate called Rogers is used. Here's a document explaining substrate matters.