Circuit Design of Transistor Electronic Filter
In many electronic circuits, especially some small signal amplifying circuits, the power supply often adds a first-stage transistor electronic filter, and its circuit structure is shown in Figure J1. If the right side of the figure is an ordinary RC filter circuit with the same effect as the electronic filter, then they have the following relationship: the left side of the figure Uec=Ib*R1+Ueb=Ib*R1
Because Iec=β*Ib (β is the DC amplification factor of the transistor)
So there is Uec=(Iec/β)*R1
The right side of the figure Uec=Rec*Iec Since the left and right figures are equivalent to each other, there is
Rec*Iec=(Iec/β)*R1 gets Rec=R1/β
The filtering performance of the two filters is generally measured by the product of R and C, so there are:
R1*C1=Rec*C1'=(R1/β)*C1'
C1=C1'/β
It can be seen from the above formula that the capacitance C1 required by the electronic filter is β times less than that required by the general RC filter. For example, if the DC amplification factor of the transistor is β=100, if the general RC filter requires a capacitance of 1000μF, if an electronic filter is used, then the capacitor only needs 10μF to meet the requirements.
More and more electronic circuits are using FETs, especially in the audio field. Unlike transistors, FETs are voltage-controlled devices (transistors are current-controlled devices), and their characteristics are more like electron tubes. It has high input impedance, large power gain, and low noise because it is a voltage control device. Its structure diagram is shown in Figure C-a.
Field effect transistor is a kind of unipolar transistor, which has only one P-N junction, and it is turned on in the state of zero bias. If a reverse bias (called gate bias) is applied between its gate (G) and source (S), under the action of a reverse electric field, the P-N becomes thicker (called a depletion region) and the channel narrows. Its drain current will become smaller, (as shown in Figure C1-b). At a certain reverse bias voltage, the depletion region will completely "pinch off" the channel. At this time, the field effect transistor enters the cut-off state as shown in Figure C-c. The reverse bias voltage at this time is called the pinch-off voltage, which is represented by Vpo. The relationship between it and the gate voltage Vgs and the drain-source voltage Vds can be expressed as Vpo=Vps+|Vgs|. Here |Vgs| is the absolute value of Vgs.
When manufacturing field effect transistors, if a thin insulating layer is added to the channel before the gate material is added, the gate current will be greatly reduced and its input impedance will also be greatly increased. Due to the existence of this insulating layer, the FET can work in a positive bias state. We call this FET an insulated gate FET, also known as a MOS FET. So there are two types of field effect transistors, one is an insulated gate field effect transistor, which can work in reverse bias, zero bias and forward bias states, and the other is a junction gate field effect transistor, which can only work in reverse biased state.
Insulated gate field effect transistors are further divided into enhancement type and depletion type. We call it a depletion type field effect transistor that is turned on under normal conditions, and an enhancement type effect transistor that is turned off under normal conditions. Enhanced field effect tube features: When Vgs=0, Id (drain current)=0, only when Vgs increases to a certain value, it starts to conduct, and a drain current is generated. The gate-source voltage Vgs when the drain current begins to appear is also called the turn-on voltage.
The characteristics of the depletion mode field effect transistor, it can work under positive or negative gate-source voltage (positive or negative bias), and there is basically no gate current on the gate (very high input resistance).
Insulated gate field effect transistors can be used in circuits applied with junction gate field effect transistors, but junction gate field effect transistors cannot be used in circuits applied with insulated gate enhanced field effect transistors.
