Basic Differential Amplifier Question

[aryajur]

differential amplifier basics

Hello,
I was wondering if I can use the normal differential amplifier as shown in the image actdiffamp as a normal opamp as shown in the circuit invamp? I mean since one terminal of the opamp is grounded, i don't think we can do that in the active differential amplifier topology, because that would cut off one input transistor??
So what additional thing do we have to do in this basic configuration for it to be connected in the configuration of a inverting amplifier as the opamp???

 

[gevy]

basic differential amplifier

Actdiffamp does not work in invamp circuit.
You must use folded cascode diff cascade with input P-channel MOS transistors for this opamp.

 

[terryssw]

how differential amplifier works

Actually it can really works for such opamp circuits. The ground potential in the invamp is just the reference potential, not necessary being the lowest potential in the circuits. Considered if you use the invamp in the lab, what supply voltage do you connected to the opamp IC? it will be usually +- 15V. In this case the 0V ground potential can work without problems.

 

[aryajur]

differential amp

So I think these opamps are designed to have a Input Common Mode Voltage equal to the defined reference voltage of the opamp, which is generally midway between the highest and the lowest potential?

 

[SkyHigh]

output voltage of basic differential amplifier

A differential amplifier serves only the input stage of an Op-amp.

An Op-amp typically has 2 to 3 or even 4 stages, depending on the application range in the market that requires them.

For differential amp to perform like a DC inverting amp, you will need to cascade a gain and an output stage at the output of the differential amp. Such could be a simple common-emitter single transistor amplifier with emitter degeneration. Inter-stage DC-blocking capacitors would be optional, in this case unncessary. Also you can cascade a simple push-pull output stage after the gain stage.

Instead of using a differential amp, you can use a simple common-emitter single transistor amp. Tap the output from collector. The inverting gain is -Rc/Re. At the base, do not use voltage divider bias. Remove R1 and R2 at the base. Such circuit can perform much like an inverting amp.

 

[aryajur]

basic differential amplifier

Hello Sky High,
Thanks for the post, but actually I knew all that, my question was mainly regarding the way we connect the opamp in the inverting amplifier configuration, how would we connect the basic differential amplifier in the configuration.

 

[chinito]

how does a differential amplifier work?

in an inverting configuration, the purpose is to get DC gain = -R2/R1 and the requirement is that opamp should act ideally (or close to ideal). if ICMR is wide range (i.e. -3V to +3V for example in 5V rail-to-rail opamp), then you should get DC gain pretty close to -R2/R1. Where do you get the impression that IF positive terminal is grounded THEN input transistor will shut off?

 

[aryajur]

question on diff amp

I was assuming before that the diffamp is ruunning on a supply +Vcc and 0 volts. In that case if a input terminal is grounded the input transistor will cut off.

 

[terryssw]

how differential amplifier work

So I think these opamps are designed to have a Input Common Mode Voltage equal to the defined reference voltage of the opamp, which is generally midway between the highest and the lowest potential?

Yes. Actually VCC, ground, VEE is just relative reference potential, the most important thing is that your information is display around your reference potential (i.e. ground), and having positive and negative quantities. In this ways you must have negative supply.

Also, suppose you just supply the opamp with +5V and 0V, then your reference ground should be placed on 2.5V. This ground is also referred to "Analog ground", which is actually the input common-mode voltage but not necessary 0V potential.

 

[radigital]

transistor based inverting differential amplifier

What is usually described as a virtual ground of 0V for opamps in literature
is usually some potential which falls in the range of the input common
mode voltage, for opamps operating with a single supply.

You can use the differential amplifier as the inv amplifier configuration,
provided you differential amplifier has a gain which is high enough.
Otherwise your inv amplifier made out of this diff amp wont
function close to the ideal inv configuration.

To prevent the cut off of one of the transistors in diff amp, you need
to bias it with some volatge which keeps it on, when diff amp is used
in inv mode.

 

[jiangwp]

differential amplifier questions

The Ground is generally the VDD/2.
So you can bias the positive port with VDD/2 to make invert amplifier work well.

 

[yeewong_su]

differential amplifier with only positive supply

Using the P_ch MOS as the input stage, you can creative the opamp

 

[masy]

inverting op amp remove r2

actually the gnd in invamp is not the gnd of the supply, it is called AC gnd, which is the common mode input in the actamp (correctly biasing the amp).

 

[mengly]

basic differential amplyfier

actually the gnd in invamp is not the gnd of the supply, it is called AC gnd, which is the common mode input in the actamp (correctly biasing the amp).

Yes. When the loop of amp gets closed, the bias voltage can almost be any value between positive supply power and negtive supply. Of couse, the middle value is better for maxium signal output swing.

 

[avinash]

diff-amp

what is your supply range