Introduction
The transistor is a current-control device. For example, control the collector-emitter current by changing the base current. In a general voltage amplification occasion, this amplification effect comes from the use of resistors to convert current into voltage. In the small-signal model, the source of the base current is the ratio of the input voltage to the base-emitter dynamic resistance rbe, which is usually kΩ. So the base current is very small, and may only be a few tenths of mA. Through the amplification of the transistor, the base current is generated between the collector and the emitter by β times. This article will introduce how transistor works in the common-emitter amplifier circuit.
1 Common-emitter Amplifier Circuit Formula
Here, take the common emitter amplifier circuit as an example:
Fig 1. Transistor Common-emitter Amplifier Circuit
△Vo=VCC-△ieRc=VCC-β△ibRc=VCC-△Vi·Rc/rbe
△Vi/rbe=△ib
Thus, the collector generates a current of β times ib:
△ie=β△ib
Furthermore, the output voltage can be obtained by the relative positive power supply potential:
△Vo=VCC-△ieRc=VCC-β△ibRc=VCC-△Vi·Rc/rbe
Thus, we can get an inverted amplified voltage signal by AC coupling and controlling the collector resistance Re. But generally the emitter will have a resistance to control the gain, so the above formula is not practical. When designing a circuit in non-extreme situations, we often hope that the circuit can work with most general-purpose transistors, avoiding the parameter that depends on component parameters such as rbe. At the same time, it is very cumbersome to consider the base current in the specific calculation. Therefore, in the general design process, the existence of the base current is ignored in an approximate calculation (In some circuits, although the base current is ignored, it is still necessary to give the base a certain current drive to make the circuit working normally). In addition, the calculation of gain is the external circuit resistance not the rbe.
2 Common-emitter Amplifier Circuit Design
The common-emitter amplifier circuit is a typical inverting amplifier, which has a wide range of applications and stable effects. First show the overall design ideas, and then explain the purpose and principles of the design in steps.
2.1 Design Steps
1)Determine the supply voltage VCC, and determine the static emitter current IE according to the frequency curve/noise curve/others.
2) Determine VE, where selects 1~2V to absorb temperature drift.
According to VE and IE, calculate the emitter static resistance RE ( IE≈IC).
3) Determine the magnification Av, and apply the relationship Av=RC/RE to calculate the static collector resistance RC. At this point, the static working point has been established.
4) Check whether the static operating point meets the requirements: positive output swing limit=VCC-IE·RC, negative output swing limit=IE·RC-VE. It is necessary to ensure that the amplified output voltage does not exceed the swing limit (usually the swing limit is larger). If RC is too large, there will be a downside clipping, so is the small RC. In addition, determine whether the power exceeds the limit: PC=VCE·IC.
5) Determine the base bias voltage as follows: According to VBE=0.6V, it is easy to get VB=VE+0.6 (divide the voltage from the power supply through the resistor). Since ib is considered to be small and negligible, the current IB0 flowing through the base voltage divider resistors (R1, R2 in the above figure) should be much larger than ib. ib is approximately calculated as IC/β, and IB0 is about an order of magnitude larger than ib, so R2=VB/IB0, R1=(VCC-VR2)/IB0.
6) Finally, determine the AC coupling capacitor value and the power supply decoupling capacitor value.
Let's first use a designed common-emitter amplifier circuit to intuitively understand the waveforms of the next parts:
Fig 2. Transistor Common-emitter Amplifier Circuit Design
As shown in the figure, the circuit uses 2SC2240 tube, 15V power supply, and the input and output are AC coupled. The output signals are as following:
Fig 3. 4-channel Signal Waves
The pale blue waveform is the input signal, selecting the sine wave of 1kHz, 1Vpp.
The green is the output signal, amplified by about 5 times, and it is inverted.
The blue is the base signal, which can be seen because the DC level is raised due to the influence of the base bias resistance.
The red is the emitter signal, which is only a fixed value away from the base signal.
2.2 Common-emitter Circuit Design
After understanding the circuit characteristics, you can design the common emitter circuit according to the design steps at the beginning of this section. The static operating point and magnification have been determined during the analysis, and the other parts are designed below.
Supply voltage: According to the swing of the output voltage, we can determine the size of the voltage. Usually the power supply voltage is larger than the output peak-to-peak value.
Transistor: Select the appropriate transistor according to the operating frequency, required power, noise level and β, etc.
Emitter current: Determine the size of the emitter current according to the frequency characteristics by consulting the device manual.
RC and RE: Determined by the emitter voltage and current, and the magnification, pay attention to review the upper and lower limits of the swing and the rated power.
Base bias resistance: VB is determined according to VE, thereby determining the voltage divider resistance of the power supply. Note that the current flowing through the voltage divider resistor should be one to two orders of magnitude higher than the base current. The base current is calculated by dividing the collector-emitter current by β.
Coupling capacitor: The AC coupling capacitor is generally 10uF. Note that the coupling capacitor of the output stage and the input impedance of the next stage will form a high-pass filter. The cutoff frequency of the filter should be handled carefully.
2.3 Circuit Performance Parameters
Through the method of AC analysis, we can obtain some characteristic parameters of the designed circuit, such as input and output impedance, magnification and so on.
Input impedance: According to AC analysis, the input impedance is the parallel value of the base bias resistance. In small signal analysis, the base emitter dynamic resistance rbe should also be connected in parallel.
Output impedance: The method to determine the output impedance is to add a load to the circuit. When the peak-to-peak output value drops to half of the no-load, the load impedance is the output value. Generally, the output impedance of the common-emitter amplifier circuit is the collector resistance RC.
Magnification: Due to the influence of the base current, the actual magnification is about 10% lower than the design value. So the design formula is more practical.
Get the full details from Transistor Common-emitter Amplifier Circuit Design Steps.
