Ortak Baz(Base) Yükselteç / Common Base Amplifier

For the Common Base Amplifier, the output of the BJT transistor is applied to the input absorber terminal when retrieving from the collector terminal. The Common Base Amplifier is another type of configuration of the bipolar connection transistor (BJT), in which the transistor's base terminal is a common terminal for both input and output signals, hence which is called the common base (CB). The common base configuration, as an amplifier, is less common than the more popular common emitter (CE) or common collector (CC) configurations, but is still used due to its unique input/output features. For the common base configuration to work as an amplifier, the input signal is applied to the emiter terminal and the output is taken from the collector terminal. Therefore, the emitter current is also the input current, and the collector current is the output current, but since the transistor is a three-layer, two-pn-connected device, it must be properly polarized to function as a common base amplifier. This is because the base-emitter connection is forward-looking. Consider the basic common base amplifier configuration below. common base Next, from the base-soil configuration, we can see that the input variables are related to the emitter current IE and base-emitter voltage VBE, while the output variables are related to collector current IC and collector-base voltage VCB. Since the emitter current, IE, is also the input current, any change in the input current will create a corresponding change in the collector stream, IC. For a common base amplifier configuration, current gain is given as Ai, iOUT/iIN, which is determined by the IC/IE formula. The current gain for a CB configuration is called Alpha, (α). Since a BJT amplifier emitter current is always larger than the collector current because it is IE = IB + IC, therefore the current gain (α) of the amplifier must be suddenly low, since IC is always less than IE with its value. IB. Thus, the CB amplifier weakens the current with typical alpha values ranging from 0.980 to 0.995. The electrical relationship between the three transistor currents, alpha, α and Beta, as shown, can be shown to give β expressions. common base

Common Base Amplifier Current Gain

common base Therefore, if the Beta value of a standard bipolar connection transistor is 100, the Alpha value is given as follows: 100/101 = 0.99.

Common Base Amplifier Voltage Gain

Since the common base amplifier cannot function as a current amplifier (Ai ≅ 1), therefore it should be capable of working as a voltage amplifier. The voltage gain for the common base amplifier is the ratio of VOUT/VIN, that is, collector voltage VC, to the emitter voltage AND ratio. In other words, VOUT = VC and VIN = AND. Since the VOUT output voltage collector resistance is developed throughout the RC, the output voltage should therefore be a function of the IC according to the Ohm Act, VRC = IC*RC. Therefore, any change in IE will have a corresponding change in ic. Then we can say that for a common basic amplifier configuration: common base

Since IC/IE is alpha, we can present the voltage gain of the amplifier as follows:

common base Therefore, voltage gain is more or less equal to the ratio of collector resistance to emitter resistance. However, within a bipolar connection transistor between the base and emiter terminals, the dynamic emitter resistance of transistors has a single pn-diode connection that leads to what is called r'e. common base

The emitter diode connection for AC input signals has an effective small signal resistance, which is given as follows: r'e = 25mV/IE, where 25mV is the thermal voltage of the pn connection, and IE is the emitter current. Thus, as the current passing through the emitter increases, the emitter resistance will decrease by a proportionate amount.

Part of the input current flows through this built-in base-emitter connection resistance to the base, as well as through the externally connected emitter resistance RE. For small signal analysis, these two resistors are connected in parallel.

Since the value to r is very small and the RE is usually much larger, usually in the kilohm (kΩ) range, the size of the amplifier's voltage gain changes dynamically with different levels of emitter current.

Thus, if the RE ≫ r, the actual voltage gain of the common base amplifier will be as follows:

common base Since the current gain is approximately equal to one in IC ≅ IE, the voltage gain equation is simplistic as common base follows: For example, if 1mA current flows through the emitter-base connection, its dynamic impedance will be 25mV/1mA = 25Ω. The volt gain for a collector load resistance of 10kΩ, av will be as follows: 10,000/25 = 400 and the more current flows from the connection, the lower the dynamic resistance and the higher the voltage gain. Similarly, the higher the value of the load resistance, the greater the voltage gain of the amplifier. However, it is unlikely that a practical common base amplifier circuit will use a load resistance greater than about 20 kΩ with typical voltage gain values from about 100 to 2000, depending on the RC value. Note that the power gain of the amplifier is approximately the same as the voltage gain. Since the voltage gain of the common base riser depends on the ratio of these two resistance values, it is concluded that there is no phase inversion between the emitter and the collector. Therefore, the input and output waveforms are "in-phase" with each other, indicating that the common base amplifier is an inverted amplifier configuration.

Common Base Amplifier Resistance Gain

One of the interesting features of the common base amplifier circuit is the ratio of input and output impedances, which leads to what is known as amplifier Resistance Gain, the main feature that makes amplification possible. We saw above that the entrance was connected to the emite and the output was taken from the collector. There are two possible parallel resistant paths between the entrance and the soil terminal. One through emitter resistance, RE to the soil and the other to the r and to the soil through the base terminal. Thus, by looking at the grounded emitter on the base, we can say: ZIN = RE|| R'e. However, dynamic emitter resistance is very small compared to r'e, RE (r'e comparedRE), built-in dynamic emitter resistance dominates the equation to r and results in a low input impedance equal to approximately r'e. Therefore, the input impedance for the common basic configuration is very low, and depending on the value of the welding impedance, the RS connected to the emitter terminal, input impedance values can vary from 10Ω to 200Ω. The low input impedance of the common base amplifier circuit is one of the main reasons for their limited application as a single-stage amplifier. However, the output impedance of the CB amplifier can be high depending on the collector resistance and the associated external load resistance, RL, which are used to control voltage gain. If a load resistance is connected to the amplifier's output terminal, it is effectively connected in parallel with the collector resistance, in which case ZOUT = RC|| RL. However, if the externally connected load resistance, RL, collector resistance is too large compared to RC, then RC will dominate the parallel equation, resulting in an average output impedance ZOUT and approximately equal to RC. Then, for a common basic configuration, the output impedance facing the collector terminal will be as follows: ZOUT = RC. Since the output impedance facing back to the collector terminal of the amplifier can potentially be very large, the common basic circuit operates as an ideal current source, taking the input current by the low input impedance and sending the current to the high output impedance side. . Therefore, the common basic transistor configuration is also referred to as a: current buffer or current tracker configuration and the opposite of the common collector (CC) configuration called voltage monitor.


In this tutorial on Common Based Amplifiers, we found that it has approximately one current gain (alpha), but also a voltage gain that can be very high with typical values ranging from 100 to 2000. We also found that the input impedance of the amplifier circuit is very low, but the output impedance may be very high. We also said that the common base amplifier does not reverse the input signal because it is a non-inverted amplifier configuration. Due to its input-output impedance features, common base amplifier editing is extremely useful in sound and radio frequency applications as a current buffer to match a low impedance source to a high impedance load, or as a single-stage amplifier as part of a system. A gradual or multi-stage configuration in which one amplifier stage is used to drive another.