# Class A or class AB preamp

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#### Havenwood

Hi, I have a simple question.

For buffer/preamp design (e.g. Diamond), use output transistor like BC547 or BD139.

What is the value of Iq that the buffer/preamp work in class A?
and when in class AB?

Use BC547 (supply +/-8V), Iq = 0.5mA is class AB and Iq = 10mA is class A?

Zout will vary with different Iq levels?

#### Mooly

Paid Member
All preamps and small signal circuitry should run in Class A.

0.5ma 10 ma or even 10ua... all can be class A depending on the load.

The load impedance should be such that the preamp never approaches class ab under maximum output.

#### AndrewT

Hi Haven,
How much capacitance is in that load.
How much current does the load//capacitance take at maximum slew rate?

From that answer you can decide whether ClassA is feasible and if it is, determine the output bias current and the quiescent current to meet that worst case demand.

If you think that worst case current cannot be met using ClassA, then push/pull ClassAB can be used to satisfy the worst case demand and you adopt a lesser ClassA capability for your source.

A single ended ClassA stage can never exceed it's output bias current.

#### Havenwood

Hi Haven,
How much capacitance is in that load.
How much current does the load//capacitance take at maximum slew rate?

From that answer you can decide whether ClassA is feasible and if it is, determine the output bias current and the quiescent current to meet that worst case demand.

If you think that worst case current cannot be met using ClassA, then push/pull ClassAB can be used to satisfy the worst case demand and you adopt a lesser ClassA capability for your source.

A single ended ClassA stage can never exceed it's output bias current.