Other Parts Discussed in Post: LM741, LMH6644, LMH6642

Vintage TV with Op-AmpIf you are as old as I am, you may remember the quintessential “Op-Amp Cook Book” by Walter Jung (I have an autographed copy) or the must have “Intuitive IC Op-Amps” by Tom Frederiksen (I also have an autographed copy).  Many of the useful circuits in both of these works employ split supply op-amps such as the LM741.  In the early days of integrated op-amps, the input and output range failed to match the supply rails.  Thus, to guarantee that the signal would not be clipped or distort, the supply voltage of choice was often +/- 15V (30V total).  As the market began to shift toward mobile, battery operated devices, the basic building block for analog was redesigned to provide rail-to-rail inputs (common mode voltage would include V- and V+) as well as outputs.  At the same time, the supply voltage was reduced in many devices below 12V. 

Since many designs of classic circuits used split supplies, ground was the common point for all signal references (+/- V).  That is, the analog signals would swing around ground.  The basic inverting and non-inverting amplifiers shown in figure 1 both use ground as the reference bias point and must have split supplies.  You may note that if you try to use the inverting topology with V- connected to ground, it will not work.  The output cannot swing below ground to drive the differential input voltage to zero.  So what do you do if you want to use newer, higher performance, lower power operational amplifiers?  Simple… read on.

FIGURE 1 - Classic Op-Amp Configurations (left is inverting, right is non-inverting)

This is a simple trick that will allow you to dive into all the classic designs and convert them to single supply operation.  Each of the classic topologies uses ground as the reference point.  To convert a design to single supply, this point must move to a new bias point that is within the operating range of the amplifier – a midpoint value.  If you are using a +12V and ground, then the center bias point would +6V.  Sounds very simple! Make a voltage divider with two resistors from the supply to ground and replace the ground connections with the new +6V divided value… right?  Maybe hang some capacitance on the output to filter the bias voltage, sounds good. Not so fast…

Depending on the topology, significant currents may be flowing in and out of the original ground point.  This means the new single supply system needs an active bias voltage control to regulate the output by both sinking and sourcing current.  A solution to that is shown in Figure 2.  Using a high power, wide bandwidth op-amp such as the LMH6642 (or LMH6644 if you want a quad), the new design will work from a single supply.  The new signal reference is now the output of the bias generator which is halfway between the rails.  Notice the series resistor on the output of the LMH6642.  This is to prevent the internal compensation from being affected which will reduce phase margin and potentially cause high frequency oscillations on the output.  Not something you want to happen for a reference voltage!

Figure 2 - Improved Single Supply System - Inverting Amplifier

Figure 2 shows that the bias circuit will drive other amplifiers (or devices) in your system (based on the drive of the LMH6642).  So if you are building filters, you can utilize the same bias point across the stages.  Hope this simple little trick helps you get over the single supply blues - it has helped me countless times!  Till next time...

Anonymous
  • Excellent question from Otto Hunt.  At first glance you would think that this would be a more simple approach.  There are several issues with using a linear regulator.  First, linear regulators are designed to source current to regulate the output voltage. The internal error amplifier controls the gain of the output stage increasing or decreasing the current to provide the correct voltage into a varying load.

    In certain op-amp topologies (such as the non-inverting configuration) current will be forced back into the bypass capacitor.  The linear regulator will simply reduce it's current flow to the output since the voltage will increase across the cap (it assumes a load drawing current). and only the leakage currents will draw down the voltage... it will not properly regulate.  

    The second issue is the bandwidth of the linear regulator's internal error amplifier... most have OK bandwidth, but nothing like the LMH6642 which has a GBW of 130MHz. It has a push-pull output stage which can sink or source current.  So as the Vbias is pushed around, the bias regluator (LMH6642) will be able to compensate by push or pulling current from the RC filter.  The LMH6642 also has around 75 mA of drive current (sink or source) so it can handle many stages in a system.

  • Would it be more appropriate to use a linear voltage regulator - a part designed to do this job?