4. Active Devices
• Active devices are different from passive devices such as resistors, capacitors and inductors. These devices are capable of changing their operational performance, may deliver power to the circuit, and can perform interesting mathematical functions.
• When doing most circuits problems we depends on idealized components. The following section will describe a number of components, their models, and how to apply them in practical circuits.
• A very common and versatile device is the operational amplifier (Op-amp). They are characterized as,
• These are available for a few cents in commercial quantities. They also come in a wide variety of packages for various applications.
4.1.1 General Details
• The schematic symbol for these devices is given below,
• Inside these devices have a very high gain amplifier that compares the inputs and gives an output that is amplified as shown by,
• When using these devices the circuit is typically set up so that both the inverting and non-inverting inputs have the same voltage, and the currents in to both of the inputs is negligible.
4.1.2 Simple Applications
• Considering that the Op-amp was originally designed to allow simple mathematical operations in circuit form, the following circuits tend to be mathematical in nature.
• A typical op-amp application is the inverting amplifier.
• We can also make non-inverting amplifiers using the following circuit,
• The integrating amplifier is a very powerful application,
• The following device is one form of differentiator using an inductor,
• A second type of circuit uses a capacitor to find the differential,
• The following circuit can be used to add inputs. If dissimilar components are used the inputs can be weighted
• We can construct an amplifier that subtracts one input from the other,
• At times we want to isolate a voltage source from an application, or add a high impedance. This can be done using a voltage follower,
• Op-amps can be used for measuring the potential across bridges.
• When used in this mode, it is probable that both inputs may have the same voltage that is not zero. The result of this common offset is that the output will drift with the common inputs. The technical measure is the Common Mode Rejection Ratio (CMRR). This is generally measured by the manufacturer, and provided in the device specifications.
• A Low pass filter will enable us to cut off the higher frequency components of an input signal,
• The basic layout of the 741 op-amp is given below for the 14 pin dip package (14C1741).
• The basic layout of the 741 op-amp is given below for the 8 pin dip package.
4.1.3 Op-Amp Equivalent Circuits
• An equivalent circuit for an op-amp is given below,
• Open loop frequency response can be estimated using the equivalent circuit below,
4.2.1 Bipolar Junction Transistors (BJT)
• Bipolar Junction Transistors (BJTs) are made with three layers of doped silicon. The layers are either doped to be positive (p-type) or negative (n-type) using low concentrations of elements mixed with the silicon.
• There are two basic types, PNP and NPN. Their names come from the sequence of doped layers in the transistor. The schematic symbols for these transistors are shown below.
• The base-emitter voltage is usually given as a constant. This junction acts much like a diode, and will on average have voltages around 0.7V.
• Transistors are highly non-linear, but they are often biased by carefully applying voltages and currents to put them in a roughly linear range.
• A designer will depend heavily upon specifications. These are often in the form of graphs for different transistor applications.
• Except for applications such as switching, most transistor configurations are used for sinusoidal signals. As a result there is usually a DC design, as well as AC.
• A common emitter configuration is shown in the figure below.
• Consider the common emitter amplifier shown. The resistors provide DC biasing to select an operating point. The capacitor Ce is used to allow the AC to bypass Re.
• To perform the design we must first bias the transistor using the curves below.