These are frequently asked MCQ on transistor in the topic such as BJT, FET and MOSFET etc. Conceptually, an ideal current mirror is simply an ideal current amplifier with a gain of -1. FET. connection, connect the sources, and bias the cross-coupled pair with a current source. ; The junction formed through the base and emitter called base-emitter junction and combination of base and collector makes the . A voltage controlled current source is a very useful circuit building block, and the Howland current source is a good example. Current-controlled device: Base current control the amount of collector current. There are two types of dependent current sources: Voltage Controlled Current Sources (VCCS) and Current Controlled Current Sources (CCCS). The circuit is very convenient and can design with a minimal number of components. The direction of positive controlling current flow is from the positive node, through the source, to the negative node of VNAM. This is not perfect, because no transistor current source is ideal. The terminals of BJT are base, emitter, and collector. In a p-n junction i.e base-emitter junction in BJT the change in voltage (Vbe) is produced only after the change in base current. There are two types of JFET namely n-channel and p-channel. If you prefer to use operational amplifiers, the Howland current pump produces a voltage-controlled current and requires only one op-amp and four resistors. A transistor is not a current-controlled current source. Note reversal in current directions and voltage signs for PNP vs. NPN! • A BJT can also be considered a voltage controlled current source because the voltage between the base and the emitter control IC and IE. (Note, that in contrast to a BJT, a MOSFET is only considered by be a voltage controlled current source since the DC gate current is zero in a MOSFET. . Current-controlled current source 1. Let's start from the beginning. Bipolar Junction Transistor Fundamentals: . 2. ESE319 Introduction to Microelectronics 2008 by Kenneth R. Laker (based on P.V. Linear Current-Controlled Voltage Sources General form: HXXXXXXX N+ N- VNAM VALUE Examples: HX 5 17 VZ 0.5K low input impedance as compared to FET Joined Jul 8, 2011 Messages 28 Helped 0 Reputation 0 Reaction score 0 Trophy points High input resistance. It stands for Bipolar Junction Transistor. Not an ideal current source: Slight dependence on VCE due to base width Topic 13 BJT.pdf - Bipolar Transistors(Bipolar Junction Transistors 1 Voltage controlled current source \u2022 \u2022 \u2022 \u2022 \u2022 \u2022 \u2022 \u2022 \u2022 \u2022 Consider Answer (1 of 17): Why is a BJT called a "current-controlled current source"? BJT operates by having a base-emitter (input) junction in . • IB = IB0 exp (VBE/Vt) and . A BJT is a voltage controlled current source (VCCS). Long answer: The confusion about whether a BJT is current controlled or voltage controlled comes from two sources. Answer. 21. No two terminals are interchange-able. a power MOSFET is a current-controlled device and BJT is a voltage-controlled device. In this project, we will build a simple transistor constant current source generator using only 4 components. BJT. . A simple high value resistor can be . Current source: A source with a fixed current (amperes) output that is able to deliver that current within a certain range of voltage. Its input (at the base) is current which controls the output collector current. VALUE is the current gain. This is not perfect, because no transistor current source is ideal. BJT Intro and Large Signal Model. The voltage value of the controlled voltage source in step (1) is the sum of all the submodules' output voltage of the bridge arm mentioned in step (2) (as shown in Eq. Voltage controlled current source: B-E voltage VBE controls collector current IC. 2.6, in BJT the output current, I_C is controlled by the base current I B. •DC Voltage sources are shorts to ground •DC Current sources are open circuits •Large capacitors are short circuits •Large inductors are open circuits 3.) NPN BJT operating in "Forward" Active Mode Conceptual View: 4 • Device acts as a voltage controlled current source • V BEcontrols I C • The BE junction is forward biased (V BE> 0) and the BC junction is reverse biased (V BC< 0 ←→V CB> 0) • The device is built such that: • The BASE region is very thin All of this current is steered between either the left or right side of the diff pair. My circuit has selectable current and voltage ranges, but these are not shown. The FET is a voltage-controlled device. They enable a fixed, or controlled current to be supplied independent of the voltage (within limits) and as such they are very useful. As shown in the Fig. Its input is a voltage or potential difference (at the gate) which controls the output current at the source. Devised by Professor Bradford Howland at MIT in the early 1960s, it is capable of both sourcing and sinking current proportional to an input voltage. The last true device to talk about is the field effect transistor (FET), specifically the metal-oxide-semiconductor type (MOSFET). Use a Thevenin circuit (sometimes a Norton) where necessary. 10 Two slightly different versions of what is known as the T model of the BJT. Only a trickle of electrons is needed to activate a FET, implying that its input impedance is very high, which in turn makes the . It turns out that a transistor is simply a device. I/O Impedance. 1. It is sometimes also called a Howland current pump. Lecture12-Small Signal Model-BJT 5 Transistor Amplifiers MOSFET Amplifier Concept MOSFET is biased in active region by dc voltage source V GS. Recall that for a MOSFET in SATURATION, the drain current i D is "controlled" by the gate-to-source voltage v GS. Now, the amplitude is controlled by I0. BJT transistors as voltage-controlled devices. It is a current controlled device because the base current controls the current flow from the emitter to collector. Current controlled current source Voltage controlled current source E T m e be e I V g i v = = = α r T C be c m V I v i g = = B T m b be I V g i v . The current is being controlled by the voltage and not the other way around. The first is that the equations we use in describing electric circuits are not definitions of one variable in terms of several others. Do not confuse this with the DC Thevenin you did in step 1 . Electronic - Parasitic oscillation in BJT booster stage of voltage controlled current source; Electrical - LM13700 Bias Current via Resistor vs. Voltage-Controlled Current Source; Electronic - Is it possible to design a linear voltage regulator which delivers constant current but adjustable output voltage They model the collector current as a current source controlled by Ie with alpha the scale factor. 3. Hybrid-π small signal model of BJT This model represents that transistor as a voltage controlled current source with control voltage v be and include the input resistance looking into the base. The BJT collector current, I C, is controlled by the much smaller base current, I B. BJT Intro and Large Signal Model. There are in general four different types of dependent sources as follows: voltage controlled current source (VCCS), current controlled current source (CCCS), voltage controlled voltage source (VCVS) and Its input (at the base) is current which controls the output collector current. (5.72) u sum = ∑ i = 1 N u SM _ i. All of this current is steered between either the left or right side of the diff pair. It will also have a potentiometer to control the current output from 1 to 100mA range. This type of circuit is commonly used in electronics to drive current-controlled devices like BJT, SCR, etc. e.g., Q-point is set at (I D, V DS) = (1.56 mA, 4.8 V) with V GS = 3.5 V Total gate-source voltage is: v GS = V GS + v gs 1 V p-p change in v GS yields 1.25 mA p-p change in i D and a 4 V p-p change in v . will only need the analog and the source library.) Select the correct statement from given answer choices. Of Transistors Dependence of current on the collector voltage - npn in active mode Characteristics of the BJT Transistor as an Amplifier Removing the signal source for dc (bias) analysis : linear operation - small signal Transistor as an Amplifier DC conditions Collector current and the Transconductance Base current Emitter current Voltage Gain Hybrid-p Model (small signal . Slower in switching It can be used as a switch for digital logic (On/O ), or as a current ampli er that is ampli ed by a constant amount referred to as (common Emitter current . Bipolar Junction Transistor MOS Transistor 1. Physics of BJTs, Fundamentals of Microelectronics, B.Razavi 13 Aug 22, 2011 #3 H. harikrishnam Junior Member level 3. BJT is. Source and Drain can be inter- In a voltage-controlled current source circuit, as the name implies, a small amount of voltage across the input will proportionally control the current flow across the output loads. On the other hand, in FET, the voltage applied between gate and source V G S controls the drain currentI_D. Terminals: The terminals of MOSFET are Source (S), Drain (D), Gate (G), and Substrate (SS). I want to add a means to limit the current while in controlled voltage mode and the voltage while in controlled current mode. Ideally the base should be a single resistor + a single source. the base-emitter current, a current controlled current source. And the actual usable voltage of a FET transfer characteristics for low current is small, so noise gets amplified. Current and voltage are generated in unison. In a voltage-controlled current source circuit, as the name implies, a small amount of voltage across the input will proportionally control the current flow across the output loads. In the previous sections, an independent current source has been presented and their value is fixed and depends only on the source's design. BJT is a current controlled device. This relation "happens" to be true, but this is not the reason. It is a very inexpensive circuit that can provide 100mA constant current source using a 5V power supply. The BJT is a current controlled device. Here is the email in its entirety: BJT. Some folks think because the Ic is tracking the Ib, the BJT is a current controlled current source (CCCS). The name "field effect" is derived from the fact that . Current-controlled device: Base current control the amount of collector current. VALUE is the current gain. 2.2 A Simple Current Source A simple BJT current source can be implemented as shown in Figure 9. Since the only path for the load For a BJT the principal is the same. The FET is a voltage-controlled device. n-channel type means the carrier type in the conducting channel is electron. Current flows due to one type of carrier (unipolar) 3. e.g., Q-point is set at (I D, V DS) = (1.56 mA, 4.8 V) with V GS = 3.5 V Total gate-source voltage is: v GS = V GS + v gs 1 V p-p change in v GS yields 1.25 mA p-p change in i D and a 4 V p-p change in v . Note reversal in current directions and voltage signs for PNP vs. NPN! The barrier voltage does not drive current. The battery source or generator energy conversion drives current. Current controlled voltage source: Hname N+ N- Vcontrol Value Current controlled current source: . The working of BJT depends on the current at the base terminal.therefore, BJT is a current controlled device. High input resistance. connection, connect the sources, and bias the cross-coupled pair with a current source. A MOSFET is known as a voltage controlled device because a voltage applied to the gate controls the flow of current between the source and drain. Close the library selection window and we will begin the construction of the following circuit. Basically, you expect the load to go from 0V to 80V . It can be used as a switch for digital logic (On/O ), or as a current ampli er that is ampli ed by a constant amount referred to as (common Emitter current . BJT operates by having a base-emitter (input) junction in . A voltage controlled current source is a very useful circuit building block, and the Howland current source is a good example. Its input is a voltage or potential difference (at the gate) which controls the output current at the source. Dependent sources. 1. ESE319 Introduction to Microelectronics 2008 by Kenneth R. Laker (based on P.V. This cannot be overemphasized. The circuit in (a) is a voltage-controlled current source representation and that in (b) is a current-controlled current source representation. In a p-n junction, such as the b-e junction in a bjt, the change in base-emitter voltage, vbe, cannot take place until after the change in base current, ib. => IS = ID! Likewise, for p-channel type, the carrier type in conducting channel is hole. Circuit Layout: Example 1: Sometimes one would like to figure out voltage, current, or power for a given circuit. The input impedance increases and transconductance is determined by the resistor mostly.
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