Designing transistor amplifier circuits requires mastering core techniques such as component selection, quiescent operating point setting, and temperature compensation. The following are the key points:
Static working point settings
The quiescent operating point directly affects circuit performance and is typically set to approximately half the supply voltage (Uce) to provide symmetrical space for signal polarity changes. For example, in a common-emitter amplifier circuit, R1 and R2 control the transistor's amplification state by adjusting the current (Uce ≈ Vcc/2). If Uce is biased towards Vcc, it tends towards cutoff; if it is biased towards 0V, it tends towards saturation.
Temperature compensation techniques
Differential amplifier circuits effectively suppress temperature drift through a symmetrical dual-transistor design. For example, when an increase in temperature causes a rise in base current, the collector currents of both transistors increase synchronously, offsetting the effect of temperature drift through the output differential signal. In practical applications, the value of the Re resistor must be appropriately selected to balance the amplification capability and the temperature drift suppression effect.
DC bias resistor adjustment
R1 and R2 act as DC bias resistors, controlling the current to determine the transistor's operating state. For example, increasing the value of R1/R2 can cause the transistor to switch from amplification to cutoff or saturation; this needs to be dynamically adjusted based on the signal amplitude to optimize the linear amplification range.
Signal coupling method
Coupling capacitors C1 and C2 enable signal transmission, utilizing the characteristic that capacitor voltage cannot change abruptly to couple AC signals. During design, the capacitor capacitance must be matched to the signal frequency to avoid phase distortion.
The core of an amplifier circuit lies in the transistor, therefore a basic understanding of transistors is crucial. Transistors can be used to construct various amplifier circuits; here we will focus on analyzing only a few common types. Figure 1 shows an example of a basic common-emitter amplifier circuit. Key aspects we need to master for amplifier circuits include: in-depth analysis of the function of each component in the circuit; a thorough understanding of the working principle of the amplifier circuit; the ability to analyze and calculate the circuit's quiescent operating point; and a full understanding of the significance and strategies for setting the quiescent operating point.
Transistors are the core components of amplifier circuits, playing a crucial role. In practical applications, amplifier circuits constructed with transistors take many forms. To better understand and apply them, we have selected the common-emitter basic amplifier circuit for detailed analysis (as shown in Figure 1). When analyzing this type of circuit, we need to master the following key points:
A thorough analysis of the specific function of each component in the circuit;
Understand and explain the amplification principle of amplifier circuits;
Master the methods for analyzing and calculating the static operating point of a circuit;
Clearly define the importance and specific methods for setting static working points.
Of these, the fourth point is particularly crucial in practical applications. Next, we will delve into the specific meaning and applications of each point using the common-emitter basic amplifier circuit shown in Figure 1 as an example.
Summary of Transistor Amplifier Circuit Design Techniques
Because the voltage across a capacitor cannot change abruptly, when an AC signal is input, the voltage at the output terminal will change accordingly, thus achieving signal coupling and transmission. It's important to note that the voltage across a capacitor is not completely immutable; rather, it gradually responds to the input signal under certain conditions.
R1 and R2 are DC bias resistors for transistor V1, providing the necessary operating conditions. The essence of DC bias is to inject stable electrical energy into the transistor to ensure its normal operation. In the circuit, the power supply must be a DC power supply to maintain operational stability. Powering the transistor through resistors is like adjusting the water flow from a faucet, controlling the current and thus determining the transistor's operating state.
The operating state of a transistor can be determined by the magnitude of Uce. When Uce is close to the power supply voltage VCC, the transistor is in a non-load state, the Ic current is small, the voltage across R2 is close to 0V, and Uce is therefore close to VCC. If Uce is close to 0V, the transistor enters saturation, the Ic current reaches its maximum value and cannot increase further. These two states are collectively referred to as the switching state. In addition, the transistor is also in an amplification state, at which point Uce is approximately half the power supply voltage. By observing the deflection of Uce, the operating trend of the transistor can be determined.
When understanding the purpose and method of setting the quiescent operating point (QOP), we need to clarify its crucial role. The QOP not only affects the operating state of the transistor but also has a profound impact on the performance and stability of the circuit. Therefore, setting the QOP appropriately is essential for optimizing circuit performance.
The core task of an amplifier circuit is to amplify the input signal and output it. This typically involves amplifying voltage, current, and power, but this article will not delve into these details. Taking a sinusoidal AC signal as an example, we focus on the change in signal magnitude, i.e., its positive or negative change. In the amplifier circuit shown in Figure 1, the setting of the quiescent operating point is crucial, and it is usually set to Uce close to half of the power supply voltage. The purpose of this setting is to provide symmetrical space for the positive and negative changes of the signal. When there is no signal input, i.e., the signal input is 0, Uce serves as a reference point, usually set to half of the power supply voltage. As the input signal increases, the currents Ib and Ic also increase accordingly, causing the voltage U2 across resistor R2 to rise, and thus Uce to decrease. Theoretically, U2 can reach a maximum of VCC, causing Uce to drop to a minimum of 0V. Conversely, when the input signal decreases, the currents Ib and Ic decrease, U2 decreases, and Uce increases accordingly. In this way, when the positive and negative changes occur within a certain range of the input signal, Uce, with 1/2VCC as the reference, can obtain a symmetrical range of change.
To design Uce to be close to half the supply voltage, we need to consider several key factors. First, Ic and Ib are the core currents of the transistor, and there is a relationship between them: Ic = β × Ib. However, determining the appropriate values for Ic and Ib is not easy for beginners, as it involves several complex factors. Generally speaking, for small-power transistors, Ic is usually set in the range of a few milliamps to a few milliamps; for medium-power transistors, it is in the range of a few milliamps to tens of milliamps; and for high-power transistors, it is in the range of tens of milliamps to a few amps. By reasonably selecting these current values, we can effectively set the quiescent operating point and optimize the performance of the amplifier circuit.
Given that VCC is 12V, then 1/2VCC is 6V. Using the law of resistance R=U/I, we can deduce that the resistance of R2 is 6V divided by 2mA, which is 3KΩ. Next, if Ic remains at 2mA, then Ib can be calculated using the formula Ib=Ic/β, where β is the key parameter. Typically, the theoretical value of β is approximately 100, therefore the value of Ib is 2mA divided by 100, which is 20μA. Further, the resistance of R1 can be calculated using the formula R1=(VCC-0.7V)/Ib, where 0.7V is the base-emitter voltage of the transistor. Substituting the known value into the formula, we get the resistance of R1 as 56.5KΩ. However, in reality, the β value of a small-power transistor may be much greater than 100, even reaching 150 to 400 or higher. Therefore, when designing using the above calculation method, the circuit may enter a saturation state.
This explains why sometimes our calculations seem correct, but the actual results are unsatisfactory. The reason behind this is the difference between theoretical calculations and actual conditions, especially affected by the β value. This difference must be considered when designing circuits to ensure stability.
In contrast, the voltage divider bias circuit shown in Figure 2 is more consistent in analysis and actual measurement. The design and calculation methods of this circuit are more reliable and stable in practical applications.
Summary of Transistor Amplifier Circuit Design Techniques
In the voltage divider bias circuit shown in Figure 2, we also set Ic to 2mA and Uce to 1/2VCC, i.e., 6V. So, how do we determine the values of R1, R2, R3, and R4? We can use the following formula for calculation: Since Uce is designed to be 6V, Ic multiplied by (R3 + R4) should equal 6V, and we can approximate Ic to be equal to Ie. Therefore, we can deduce that the sum of R3 and R4 is 3KΩ. In practice, R4 is usually 100Ω, while R3 is generally chosen to be 2.9KΩ. However, it should be noted that since 2.9KΩ resistors are not available in the E24 series, we usually choose a closer value of 2.7KΩ, which does not cause a significant difference. Furthermore, the voltage across R2 is equal to Ube plus UR4, i.e., 0.7V plus 100Ω multiplied by 2mA, approximately 0.9V.
Next, we set Ic to 2mA and use the theoretical value of β (100) to calculate Ib, obtaining Ib = 2mA/100 = 20μA. Simultaneously, we need to estimate the current flowing through R1, which is typically about 10 times Ib, or 200μA. Then, we can use the resistance law to calculate the resistance values of R1 and R2. We find that R1 is approximately 56KΩ, while the resistance of R2 depends on the actual value of β. Considering that the actual value of β may be much greater than 100, the actual value of R2 might be 4.7KΩ.
Temperature Compensation and Stability ◉ Temperature Compensation
Finally, let's understand the stable operating principle of the amplifier circuit. Increased temperature leads to increased current. Therefore, by varying the feedback resistor R4 and the bias voltage Ub, this increase can be suppressed to stabilize the collector current. Specifically, when an increase in temperature causes an increase in the collector current Ic, the emitter current Ie will also increase accordingly. Since the voltage across R4 is proportional to Ie (i.e., IeR4 increases), and Ub is provided by the voltage divider resistor and remains relatively stable, Ube (Ub minus IeR4) will decrease accordingly. This change leads to a decrease in the base current Ib, thereby suppressing further increases in the collector current Ic, helping to stabilize the DC operating point of the collector current, and thus mitigating the negative impact of temperature rise on the circuit.
◉ Current negative feedback bias circuit
By appropriately selecting the bias resistor and DC operating point, the current negative feedback bias circuit is highly regarded for its excellent temperature stability. This bias circuit is widely used in amplifier circuits. To reduce AC energy loss across R4, capacitor C3 can be added to bypass the AC signal to ground, thereby improving the AC gain of the amplifier circuit. Therefore, this design ensures the stable and reliable operation of the amplifier circuit.
In conclusion, to fully leverage the advantages of transistors, it is essential to deeply understand and master the working principles and application techniques of the three basic amplifier configurations mentioned above.
