Single supply op amp to have a bias of VCC/2 to get max output swing? - Page 1
Forum section: electronics / beginners

author: czk9527
author_rank: Contributor
timestamp: April 27, 2016, 12:08:01 pm
content: Hey guys,I was building a differential amplifier using LM324, with a single supply of 5V. The circuit diagram is attached. The resistor are 2.7k ohms(I know I should use high resistance, but I only have 2.7k ohms 0.1% resistors), so it's unity gain.When I applied two DC inputs with a voltage difference of less than around 0.7V, the output could NOT go below 0.7V. It had no problems if the voltage difference is larger than 0.7V. I tried to increased the power supply to 10V, still no luck. Then I tried to switch it to dual supply, the problem's gone.So I did some research about it and came across one of the Texas Instruments Application Report. It says "Since this circuit has a single supply voltage (5 V), the op amp must have a bias of VCC/2 to get the maximum voltage swing at the output. "(the page is attached). I don't really understand what that means. It looks like it's got something to do with my problem. Would it because I didn't have this Vcc/2 bias?Can someone explain this to me, please?Thanks in advance

author: AndyC_772
author_rank: Super Contributor
timestamp: April 27, 2016, 12:19:25 pm
content: The output voltage of an op-amp can't ever go below its ground, or up beyond its positive supply rail. In many cases, the output can't even get near the supply rails, due to functional limitations of the op-amp's design.Your circuit is a classic instrumentation amplifier design, for which the output voltage is equal to some constant * (V2-V1).This is fine provided V2 is significantly greater than V1, because the output voltage of the final op-amp is well above ground.However, if V2 is close to V1, or more negative than V1, then the final op-amp output goes outside its usable range.In single supply applications we overcome this by biasing signals about some point which isn't ground; typically VCC/2 is used because this allows the maximum possible swing in either direction before hitting the supply rails.In the case of the instrumentation amplifier, this can be easily achieved by substituting the bottom right resistor R for a pair of resistors, each of value 2R. One of these connects to GND as shown, and the other connects to VCC. These two resistors are equivalent to a single resistor of value R, connected to a point at VCC/2.If we had a constant voltage supply equal to VCC/2 available, then we could simply connect the single resistor R to this point instead of to ground, but we don't - hence the need for two resistors as an equivalent circuit.Once you've made this modification, the output should equal VCC/2 for V1=V2, and can move either way depending on the inputs.You'll still be limited by how close to VCC and GND the LM324 can get. If you need a greater output swing, swap the LM324 for a better op-amp with "rail-to-rail" outputs.

author: Andy Watson
author_rank: Super Contributor
timestamp: April 27, 2016, 12:25:06 pm
content: Check the datasheet - in particular look at the output arrangement. The output can only actively pull down to one transistor Vce sat. plus Veb above the negative supply rail (VEE). The output voltage will go lower but it is only pulled down by a constant current source (50\$\mu\$A IIRC).

author: czk9527
author_rank: Contributor
timestamp: April 27, 2016, 12:43:21 pm
content: Thank you both AndyC and Andy W. You guys really informative

author: Lee Leduc
author_rank: Regular Contributor
timestamp: April 27, 2016, 02:50:20 pm
content: Op Amps for Everyone, 2002 edition is still available free online. Later editions apparently are no longer free. It has a good section about designing single supply op amp circuits. I originally got my copy from TI as a free download.Googled "op amps for everyone pdf download 2002" and this was the first site Referenced to TI.https://www.google.com/search?q=op+amps+for+everyone+pdf+download+2002&ie=utf-8&oe=utf-8orhttp://forum.allaboutcircuits.com/attachments/slod006b-op-amps-for-everyone-pdf.75480/

author: danadak
author_rank: Super Contributor
timestamp: April 27, 2016, 03:25:33 pm
content: Resistor matching, even Aol matching is everything in getting good CMR outof an IA.https://www.dropbox.com/s/plck7e95v7pw33c/CMR%20Analysis%20IA.pdf?dl=0Attached a labview run looking at Aol effects.In general a laser trimmed IA from someone like Analog Devices is the way to goif you need stable high performance CMR, IA configuration.Regards, Dana.