Introduction

In this chapter, we will give a brief introduction to the biasing of amplifiers or amplifier stages. We will start with a definition of the term biasing:

Definition

Definition

Biasing is the application of a collection of techniques for fixing the electrical operating conditions of electronic devices, and deriving the required bias voltage and current sources from the power supply voltage(s).

In Chapter {ref}`Chap-ampchar` we stated that the ideal behavior of an
amplifier can be characterized by three curves:



1. The $v-i$ characteristic of the input port

2. The $v-i$ characteristic of the output port

3. The input-output characteristic.


Since amplifiers are intended to behave in a linear, stationary and
instantaneous manner, these characteristics should all be straight lines that
pass the origin (see {numref}`fig-vamp-chars`). These characteristics
should not change over time. Conceptually, this is true, but in practice, we
may need to add offset to these characteristics to shift them out of the origin.

```{figure} Figures/micAmpADCsystem.svg
---
name: fig-micAmpADCsystem
width: 700px
---
Simple audio digitizing system. Only aspects relevant to amplifier biasing are shown.

Let us, for example, consider the audio digitizer shown in fig-micAmpADCsystem. The amplifier must adapt the microphone output voltage range to the input voltage range of the analog to digital converter (ADC). Let us assume the microphone is a electrodynamic type. An electrodynamic microphone comprises a membrane that is connected to a voice coil. This voice coil is placed in a magnetic field, and any motion caused by a change in air pressure on the membrane induces a voltage at the output of the voice coil. Such a microphone can also act as a telephone.

Fig. 294 The amplifier in the system from fig-micAmpADCsystem must provide voltage amplification and level shift: A. The probability density functions of the source voltage and the load voltage B. The \(0.2V_{pp}\) source voltage needs to be converted into a \(4V_{pp} \) load voltage. This requires voltage amplification. C. The mean value of the source voltage equals zero, while that of the load should be \(2V\). This requires a level shift function.

When driven from a signal voltage, the membrane causes a variation of the air pressure, which can be experienced as sound. Similarly, a DC current through the voice coil brings the membrane out of its quiescent position. Such a DC bias for a dynamic microphone is undesirable.

Hence, the DC input bias current of the amplifier, as introduced in sections Modeling of the static nonlinear behavior and Static nonlinear behavior, is not allowed to flow through the microphone. This can be achieved by using AC coupling between the microphone and the amplifier. Alternatively, this bias current may be provided by a bias current source, as discussed in section Modeling of the static nonlinear behavior.

Another important aspect of biasing is that the microphone produces a bipolar signal with an average value of zero, while the ADC can only accept input voltages between \(0\) to \(4\)V. In the absence of a signal, the ADC input voltage should equal its midrange value: \(2\)V. Hence, aside from raising the signal level from \(0.2\)V\(_{\text{pp}}\) to \(4\)V\(_{\text{pp}}\), we need to change the zero-signal voltage level from \(0\)V\ to \(2\)V. Such an addition of an offset voltage is often referred to as application of a voltage level-shift , or simply a level shift.

Fig. 294 illustrates the combination of the signal amplification function and the level shift function that both have to be performed by the amplifier.

This chapter

The concept of DC coupling , AC coupling and the application of voltage level shifts and bias current sources, as well as the selection of the power supply voltages, will be discussed in section Basic techniques.

Basic techniques, such as, AC coupling and application of level shifts do not always provide sufficiently accurate biasing. Changes in the power supply voltages, temperature variations and device tolerances may unacceptably affect the biasing of the amplifier and that of the source and the load. If the variations in the operating voltages and currents are too large, error reduction techniques such as compensation, negative-feedback biasing, auto-zero techniques and modulation techniques

Such techniques are applied in so-called {chopper stabilized amplifiers}.

need to be applied to improve the stability of the quiescent operating point. The application of error reduction techniques for improvement of the stability of the biasing will be discussed in section Application of error reduction techniques.

A separate section will be devoted to common-mode biasing. In section Common-mode biasing, we will discuss techniques for fixing common-mode voltages and/or currents in balanced amplifiers to their desired values.