dislocations
- Feb 21, 2012
- 25
- Joined
- Feb 21, 2012
- Messages
- 25
I'd appreciate any help! Trying to breadboard an LM741 filter but don't understand the concept of two different power supplies.
I like to think of the other way around. All opamps work off a single supply. If they did not, why don't they have 3 power connections? The difference between dual supply and single supply opamps is in how close to V- the inputs and output can go. Older one like the 741 can only get within about 2V of each rail. So it is pretty useless with a single supply IF the output needs to be near ground. On the other hand, if an application will always have the inputs and and output in a range of say 2V to V+-2V, the 741 will work as a single supply opamp.First, realize that all voltage-input operational amplifiers are inherently dual-supply - they expect to have a positive supply rail, a negative supply rail, and a ground halfway between the positive and negative rails. This is true even for op amps advertised as working with a single power supply. The most common method to obtain a positive rail, negative rail, and ground is with a positive power supply and a negative power supply with their grounds tied together. So how can an op amp work with a single supply? You just fake it out with a virtual ground. When using a single supply, the positive power becomes the positive rail while the power ground becomes the negative rail. A resistor voltage divider provides a virtual ground voltage halfway between the positive and negative rails. Since the input bias current of the '+' op amp terminal (the one usually grounded) is about 100 nA, it is reasonable for the divider resistors to be in the range of 10K. This works fine for the op amp input which is a high impedance load, but it will not work for high current op amp outputs where the signal is ground referenced, i.e., when the output is at the virtual ground voltage, then the output current is zero. For that you need a dual supply setup where the ground is a low impedance point.
Okay, then, tell my what is different about single supply op amps that allows them to work without a low-impedance virtual ground?@BobK: "All opamps work off a single supply." This is true, but not all op amp circuits will work correctly off a single supply, that is, unless you devise a low impedance virtual ground which you get by default with a dual power supply.
Just re-thought my idea...........<snip>....... I'll revisit the op-amp method at a later date. Many thanks again all!!!!!!!!!