# What is difference between dc gain and ac gain ?

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#### ahmedsaber

##### Junior Member level 1
Hi Everyone

What is difference between dc gain and ac gain ?

It is said that op amp gain up dc and ac. lets say op amp gain is 10.
Does it mean that an 10mVac fluctuating on a 10Vdc, would output a 100mVac fluctuating on a 100Vdc?
It doesnt sound so logical to have 100Vdc. If it dc gain does not gain up that way, how does it work?

#### D.A.(Tony)Stewart

It is the same , that is until the Gain * f product (GBW) is reached.

If OA has GBW of 1MHz then gain of 10 is 100kHZ BW.

If usingg +/-10V supply (and has Rail to Rail saturating outputs) , then with a gain of 10, then the max. linear input range is <1V input MAX for either AC or DC or AC+DC

##### Super Moderator
Staff member
Frequently there is a DC component, which the AC signal rides on.

The op amp's internal circuitry can amplify the AC portion only, without changing the DC level.

#### D.A.(Tony)Stewart

One could also ask the question, what is the difference between a single ended amp and a difference amp

ANS: Nothing much except perhaps common mode rejection. The single ended amp always has a DC reference for differential DC gain.

#### KlausST

##### Super Moderator
Staff member
Hi,

DC gain is the amplification (multiplication) of DC input voltage (at an OPAMP)
A = U(out) / U(in); all under DC conditions.

AC gain ist the same as with DC, but you need to know what (AC) frequency.

OPAMP circuits (with feedback = closed loop) that have different gain at different frequencies are called "filters".
For example a receiver for an IR remote control has low DC gain (surrounding light) and high AC gain (high frequency remote control signal).

An OPAMP without feedback (called "open loop") has extremely high DC gain. It is about constant from 0Hz up to some 10 or 100Hz then it decreases with increasing frequency.
This is caused by the internal circuit and can not be avoided.

Klaus

ICYou

### ICYou

Points: 2

#### crutschow

Amps with DC gain will amplify both DC and AC up to the frequency response and output voltage limits of the amplifier.
Thus if you had a 1Vac signal within the bandwidth of the amplifier with a 1Vdc bias and you had an amp with a gain of two, then the output would be a 2Vac signal with a 2Vdc bias.

Amps with AC gain only, ignore any DC bias and only amplify the AC portion of the signal. This can be achieved by coupling the signal through a series capacitor. The capacitor blocks any DC but passes the AC.

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