NationMaster - Encyclopedia: Common base

In electronics, a common base (also known as a grounded-base) circuit is a basic bipolar transistor amplifier topology, commonly used as a current buffer or voltage amplifier. In this circuit arrangement, the base node of the transistor is connected to ground (or more often, a small signal ground), the emitter node acts as the input and the collector node is used as the output. The FET equivalent of the common base is the common gate. Image File history File links NPN_common_base. ... Image File history File links NPN_common_base. ... R1, R2 and RE provide bias in this common emitter amplifier (CEVDB) circuit configuration Biasing in electronics is the method of applying a predetermined voltage and current to the junction of a transistor to set the appropriate quiescent point. ... This article is about the engineering discipline. ... The schematic symbols for pnp_ and npn_type BJTs. ... The term amplifier as used in this article can mean either a circuit (or stage) using a single active device or a complete system such as a packaged audio hi-fi amplifier. ... A buffer amplifier (sometimes simply called a buffer) is one that provides impedance transformation from high to low between one circuit and another. ... Large power N-channel field effect transistor The field-effect transistor (FET) is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel in a semiconductor material. ... Common gate amplifier A common-gate amplifier is one of the common configurations of FET electronic amplifier. ...

1 Applications
2 Using equivalent circuits
- 2.1 Current follower
  - 2.1.1 Current follower characteristics
- 2.2 Voltage amplifier
  - 2.2.1 Voltage amplifier characteristics
3 See also
4 External links

Applications

This arrangement is not very common in low frequency circuits, where it is usually employed for amplifiers that require an unusually low input impedance, for example to act as a preamplifier for moving-coil microphones. However, it is popular in high frequency amplifiers, for example for VHF and UHF, because its input capacitance does not suffer from the Miller effect, which degrades the bandwidth of the common emitter configuration, and because of the relatively high isolation between the input and output. This high isolation means that there is little feedback from the output back to the input, leading to high stability. The input impedance or sometimes loading impedance of a circuit or electronic device is the impedance actually experienced by a signal which is connected to its input. ... An example of a typical high-end stereo preamplifier. ... Microphones redirects here. ... Very high frequency (VHF) is the radio frequency range from 30 MHz to 300 MHz. ... This article is about the radio frequency. ... In electronics, the Miller effect describes the fact that a capacitance between input and output of an amplifier is multiplied by (with is the voltage gain) in a electrical circuit. ... Common emitter amplifier, voltage divider bias (CEVDB) circuit configuration A common emitter is a type of electronic amplifier stage based on a bipolar transistor in series with a load element such as a resistor. ...

This configuration is also useful as a current buffer since it has a current gain of approximately unity (see formulas below). Often a common base is used in this manner, preceded by a common emitter stage. The combination of these two form the cascode configuration, which possesses several of the benefits of each configuration, such as high input impedance and isolation. A cascode is an arrangement of electronic active devices that combines two amplifier stages for increased output resistance and avoiding the Miller effect, resulting in high gain with increased bandwidth. ...

Using equivalent circuits

Current follower

Figure 2 shows the common-base amplifier used as a current follower. The signal is provided by an AC Norton source (current i_S, Norton resistance R_S) for signal at the input, and has a resistor load R_L at the output. Although the hybrid-pi model can be used directly in this circuit to find useful parameters like the input resistance R_in and output resistance R_out, a general technique that is easier to use is based upon two-port networks. For an application like this one where current is the output, an h-equivalent two-port is selected (Figure 3) because it uses a current amplifier in the output port. (Note: By convention, positive currents are taken as directed into the port.) The h-equivalent is found for the simple case with R_L, R_S = ∞. That makes the analysis easy. Then the load and source impedance are added to the equivalent circuit to find R_in, R_out, and the actual current gain of the buffer. Nortons theorem for electrical networks states that any collection of voltage sources and resistors with two terminals is electrically equivalent to an ideal current source, I, in parallel with a single resistor, R. For single-frequency AC systems the theorem can also be applied to general impedances, not just... The hybrid-pi model is a popular circuit model used for analyzing the small signal behavior of transistors. ... Example two-port network A two-port network (or four-terminal network, or quadripole) is an electrical circuit or device with two pairs of terminals. ...

Note: In the following table, r_E = r_π / (β + 1). Tabulated quantities are for the simplified case R_L, R_S = ∞. Parallel lines (//) indicate components in parallel. Electrical circuit components can be connected together in one of two ways: series or parallel. ...

Figure 2: Bipolar current follower biased by current source I_E and with active load I_C

Figure 3: H-equivalent two-port showing independent variables I₁ and V₂

Figure 4: Bipolar current follower with signal from AC current source i_S and loaded by a resistor load R_L; emitter, base and collector indicated by arrows

Table 1	Definition	Expression	Approximation
Current gain	$\text{[math]}$ $A_i =h_{21} = begin{matrix} {i_mathrm{out} over i_mathrm{in} }end{matrix} Big\|_{v_{out}=0}$	$\text{[math]}$ $begin{matrix} - frac {frac {beta }{beta+1}r_O +r_E} {r_O+r_E} end{matrix}$	$\text{[math]}$ $begin{matrix} - frac {beta }{beta+1}end{matrix}$
Input resistance	$\text{[math]}$ $R_{11} = h_{11}= begin{matrix} frac{v_{in}}{i_{in}}end{matrix} Big\|_{v_{out}=0}$ $\text{[math]}$	$\text{[math]}$ $r E / / r O$	$\text{[math]}$ $r E$
Output resistance	$\text{[math]}$ $R_{22} = begin{matrix} frac{1}{ h_{22}} = begin{matrix} frac{v_{out}}{i_{out}}end{matrix} Big\|_{i_{in}=0} end{matrix}$	$\text{[math]}$ $(β + 1)(r O + r E)$	$\text{[math]}$ $(β + 1) r O$
Reverse voltage gain	$\text{[math]}$ $B_v = h_{12} = begin{matrix} {v_mathrm{in} over v_mathrm{out} }end{matrix} Big\|_{i_{in}=0}$	$\text{[math]}$ $begin{matrix} frac {r_E} {r_E+r_O} end{matrix}$	$\text{[math]}$ $begin{matrix} frac {r_E} {r_O} end{matrix}$ << 1

The negative sign for the current gain reflects that the exit current flows outward from the port, which by convention is a negative current direction. Image File history File links Size of this preview: 586 Ã— 599 pixelsFull resolutionâ€Ž (753 Ã— 770 pixels, file size: 61 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... Image File history File links Size of this preview: 586 Ã— 599 pixelsFull resolutionâ€Ž (753 Ã— 770 pixels, file size: 61 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... Image File history File links Size of this preview: 800 Ã— 178 pixelsFull resolutionâ€Ž (949 Ã— 211 pixels, file size: 25 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... Image File history File links Size of this preview: 800 Ã— 178 pixelsFull resolutionâ€Ž (949 Ã— 211 pixels, file size: 25 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... Image File history File links Size of this preview: 800 Ã— 306 pixelsFull resolutionâ€Ž (1,107 Ã— 424 pixels, file size: 51 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... Image File history File links Size of this preview: 800 Ã— 306 pixelsFull resolutionâ€Ž (1,107 Ã— 424 pixels, file size: 51 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. ... The input impedance or load impedance of a circuit or electronic device is the impedance actually experienced by a signal which is connected to it. ... The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ... In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. ...

Figure 4 shows the h-equivalent circuit with terminations added; it is driven with an AC Norton source (current i_S, Norton resistance R_S) for signal at the input, and has a resistor load R_L at the output. Supposing that the reverse (backward) voltage is negligible, the voltage source B_v v₂ becomes a short circuit. With that approximation, the current i₁ is found by current division as: Nortons theorem for electrical networks states that any collection of voltage sources and resistors with two terminals is electrically equivalent to an ideal current source, I, in parallel with a single resistor, R. For single-frequency AC systems the theorem can also be applied to general impedances, not just... The Current Divider (or division) rule (Sometimes referred to a CDR) is used to find the current flowing through an impedance or other circuit when it is connected in parallel with another impedance. ...

$i_1 = begin{matrix} frac {R_S} {R_S+R_{11}} i_S end{matrix}$ .

Likewise, the load current by current division is:

$i_L = begin{matrix} frac {R_{out}} {R_L+R_{22}}(- A_i) i_1 end{matrix}$ .

Combining these relations the overall (or loaded) current gain is:

$A_{i( mathrm{loaded})} = begin{matrix} frac {i_L} {i_S}end{matrix} = begin{matrix} frac {R_{22}} {R_L+R_{22}}(- A_i ) begin{matrix} frac {R_S} {R_S+R_{11}} end{matrix} end{matrix}$ ,

which indicates unity gain is obtained provided R_L << R₂₂ ≈ (β+1)r_O, and provided R_S >> R₁₁ ≈ r_E. If R_S becomes less than or comparable to r_E, then the neglect of the backward voltage is not a good approximation, and representation of the driver by a Norton source should be replaced by representation with a Thévenin source. The common base circuit stops behaving like a current follower (current buffer) and behaves like a voltage amplifier. For that application, a g-equivalent circuit provides easier analysis and more insight. Thevenins theorem for electrical networks states that any combination of voltage sources and resistors with two terminals is electrically equivalent to a single voltage source V and a single series resistor R. For single frequency AC systems the theorem can also be applied to general impedances, not just resistors. ...

Current follower characteristics

Figure 4 shows the h-equivalent circuit with terminations added; it is driven with an AC Norton source (current i_S, Norton resistance R_S) for signal at the input, and has a resistor load R_L at the output. Using the h-equivalent circuit including the reverse (backward) voltage, the following characteristics are obtained for the current amplifier application when the load and driver are attached as in Figure 4: Nortons theorem for electrical networks states that any collection of voltage sources and resistors with two terminals is electrically equivalent to an ideal current source, I, in parallel with a single resistor, R. For single-frequency AC systems the theorem can also be applied to general impedances, not just...

Note: Notation r_E = r_π / (β + 1), and identity g_mr_π = β. Symbols A_i, B_v, R₁₁ and R₂₂ refer to the previous table, Table 1.

Table 2	Definition	Expression	Approximate expression	Conditions
Current gain	${A_{i}(mathrm{loaded})} = {i_mathrm{L} over i_mathrm{S}}$	$(- A_i) begin{matrix} frac {R_{22}} {R_{22}+R_L} frac {R_S} {R_S+R_{11}+B_v A_i (R_L//R_{22})}end{matrix}$	$\text{[math]}$ $begin{matrix}frac {beta}{beta+1} frac {R_S} {R_S+r_E} end{matrix}$	$(β + 1) r O$ >> $R L$ ; $r E < < r O$
Input resistance	$R_mathrm{in} = frac{v_{S}}{i_{1}}$	$\text{[math]}$ $R 11 - B v$ $A i$ $(R 22 / / R L)$	$\text{[math]}$ $r E$	$\text{[math]}$ $(β + 1) r O$ >> $R L$
Output resistance	$R_mathrm{out} =- frac{v_{2}}{i_{L}}$	$\text{[math]}$ $begin{matrix} R_{22} // frac {R_{11}+R_S} {-A_i B_v} end{matrix}$	$\text{[math]}$ $(( beta +1)r_O) // (( begin{matrix} 1+ frac { R_S} {r_E} end{matrix} ) r_O )$	$(β + 1) r O$ >> $R L$ ; $r E < < r O$

For R_S values in the vicinity of r_E the amplifier is transitional between voltage amplifier and current buffer. For R_S values below r_E the driver representation as a Norton source should be replaced by representation with a Thévenin source. The common base circuit stops behaving like a current follower (current buffer) and behaves like a voltage amplifier . For that application, a g-equivalent circuit provides easier analysis and more insight. In electrical circuit theory, ThÃ©venins theorem for linear electrical networks states that any combination of voltage sources, current sources and resistors with two terminals is electrically equivalent to a single voltage source V and a single series resistor R. For single frequency AC systems the theorem can also...

Voltage amplifier

For the case when the common-base circuit is used as a voltage amplifier, the circuit is shown in Figure 5. This circuit is like Figure 2, but the signal now is provided by an AC Thévenin voltage source v_S with Thévenin resistance R_S. The load again is a simple load resistor R_L at the output. The signal is coupled to the amplifier with a large coupling capacitor to avoid disturbing the DC bias of the amplifier. Both the load and the input coupling capacitors are considered large enough to act as short-circuits at frequencies of interest. Thevenins theorem for electrical networks states that any combination of voltage sources and resistors with two terminals is electrically equivalent to a single voltage source V and a single series resistor R. For single frequency AC systems the theorem can also be applied to general impedances, not just resistors. ...

In this case of voltage amplification, analysis is simplest if the g-equivalent two-port is used, as shown in Figure 6, because the g-equivalent has a voltage amplifier in its output section. Example two-port network A two-port network (or four-terminal network, or quadripole) is an electrical circuit or device with two pairs of terminals. ...

Figure 5: Bipolar voltage amplifier biased by current source I_E and with active load I_C

Note: Tabulated quantities are for the simplified case R_L = ∞, R_S=0. Image File history File links Size of this preview: 494 Ã— 600 pixelsFull resolutionâ€Ž (611 Ã— 742 pixels, file size: 55 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... Image File history File links Size of this preview: 494 Ã— 600 pixelsFull resolutionâ€Ž (611 Ã— 742 pixels, file size: 55 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ...

Table 3	Definition	Expression
Voltage gain	$\text{[math]}$ $A_v =g_{21} = begin{matrix} {v_mathrm{out} over v_mathrm{in} }end{matrix} Big\|_{i_{out}=0}$	$\text{[math]}$ $g m r O + 1$
Input resistance	$\text{[math]}$ $R_{11} = begin{matrix} frac{1}{ g_{11}} end{matrix}= begin{matrix} frac{v_{in}}{i_{in}}end{matrix} Big\|_{i_{out}=0}$ $\text{[math]}$	$\text{[math]}$ $r π$
Output resistance	$\text{[math]}$ $R_{22} = g_{22} = begin{matrix} frac{v_{out}}{i_{out}} Big\|_{v_{in}=0} end{matrix}$	$\text{[math]}$ $r O$
Reverse current gain	$\text{[math]}$ $B_i = g_{12} = begin{matrix} {i_mathrm{in} over i_mathrm{out} }end{matrix} Big\|_{v_{in}=0}$	$\text{[math]}$ $- 1$

Figure 6: G-equivalent two-port showing independent variables V₁ and I₂

In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. ... The input impedance or load impedance of a circuit or electronic device is the impedance actually experienced by a signal which is connected to it. ... The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ... In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. ... Image File history File links Size of this preview: 800 Ã— 186 pixelsFull resolutionâ€Ž (926 Ã— 215 pixels, file size: 26 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... Image File history File links Size of this preview: 800 Ã— 186 pixelsFull resolutionâ€Ž (926 Ã— 215 pixels, file size: 26 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ...

Voltage amplifier characteristics

Figure 7 shows the g-equivalent circuit with terminations added; it is driven with an AC Thévenin source (voltage v_S, Thévenin resistance R_S) for signal at the input, and has a resistor load R_L at the output. Using the g-equivalent circuit including the reverse (backward) current, the following characteristics are obtained for the voltage amplifier application when the load and driver are attached as in Figure 7: Thevenins theorem for electrical networks states that any combination of voltage sources and resistors with two terminals is electrically equivalent to a single voltage source V and a single series resistor R. For single frequency AC systems the theorem can also be applied to general impedances, not just resistors. ...

Figure 7: G-equivalent two-port for simple tabulated case with Thévenin driver and load resistor added

Note: Notation r_E = r_π / (β + 1), and identity g_mr_π = β.

Table 4	Definition	Expression	Approximate expression	Conditions
Voltage gain	${A_{v}(mathrm{loaded})} = {v_mathrm{L} over v_mathrm{S}}$	$begin{matrix} frac {(g_m r_mathrm{O}+1)R_L r_{pi}} {(g_m r_O+1)r_{pi}R_S+(r_{pi}+R_S)(R_L+r_O)} end{matrix}$	$\text{[math]}$ $begin{matrix}frac {beta}{beta+1} frac {R_L} {R_S+r_E} end{matrix}$	$r O$ >> $R L$ ; $g m r O$ >> 1
Input resistance	$R_mathrm{in} = frac{v_{1}}{i_{S}}$	$\text{[math]}$ $begin{matrix} frac {r_E (r_O+R_L)} {r_E+r_O+frac {R_L} {beta +1}} end{matrix}$	$\text{[math]}$ $r E$	$\text{[math]}$ $r O$ >> $R L$
Output resistance	$R_mathrm{out} = frac{v_{L}}{i_{2}}$	$\text{[math]}$ $[1 + g m (r π / / R S)]$ $r O + (r π / / R S)$	$\text{[math]}$ $r O$	$\text{[math]}$ $R S$ << $r E$ ; $r O$ >> $(r π / / R S)$

For R_S values in the vicinity of r_E the amplifier is transitional between voltage amplifier and current buffer. For R_S values above r_E, the driver representation as a Thévenin source should be replaced by representation with a Norton source. The common base circuit stops behaving like a voltage amplifier and behaves like a current follower (current buffer). For that application, an h-equivalent circuit provides easier analysis and more insight. In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. ... The input impedance or load impedance of a circuit or electronic device is the impedance actually experienced by a signal which is connected to it. ... The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ... Nortons theorem for electrical networks states that any collection of voltage sources and resistors with two terminals is electrically equivalent to an ideal current source, I, in parallel with a single resistor, R. For single-frequency AC systems the theorem can also be applied to general impedances, not just...

External links

Basic BJT Amplifier Configurations
NPN Common Base Amplifier — HyperPhysics

Category: Single-stage transistor amplifiers HyperPhysics is an educational resource on Physics topics. ...

Results from FactBites:

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