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Voltage & Current source

  1. Apr 7, 2005 #1
    I have learnt that transistors are current source.

    a) I want to know what is the difference between current source and voltage source?
    b) Can't voltage source also be a current source?
    c) When we use a current source and when a voltage source

    Thanking you in advance. :rofl:
  2. jcsd
  3. Apr 7, 2005 #2
  4. Apr 7, 2005 #3
    MOSFETS rely on a voltage difference to Pinch, or expand the channel through which cureent can flow. The gate on a MOSFET is electrically insulated from the source to drain channel. Basically, this junction acts like a capacitor where an e-field controls the current flow.

    Now, a BJT actually requires a current flow through the base to produce a current from/to the collector to/from the emitter--confusing... Here lets look at a NPN transistor. A 2N2222 is a very common variety so if you want to, you can easily find the data sheet and read a little about it... The 2N2222 has a gain (hfe) of about 100 with a base current flow of 20mA I believe(I don't have the data sheet in front of me). This means that current must flow from the collector through the base before current is allowed to flow from the collector to the emitter. This is because a BJT is really two diodes glued together. One of the diodes conducts as youd expect(in the forward direction) while the other conducts in the reverse direction. This is all semiconductor theory and gets pretty indepth. If you want you can look into majority and minority carriers and secondary current flow paths and internal capacitance or the 12 current flow paths though a BJT; however, it's probably a little beyond what you wnated to know.

    One thing you have wrong is that a BJT( aka transistor) is a current source. It's not really a source but rather it is current controlled device. Your power supply is the actual current source(the answer to question b). Your BJT just controls the flow of current through itself when a small current is applied to the base. Now, you have set these things up correctly to get them to work. For instance, You need a higher voltage on the base of a NPN than on the collector in order to fully turn on an NPN transistor (look into H-Bridges and you'll see most use a P-type device one the upper legs and an N-type on the lower). PNP's are better at acting as current sources (you actually ground out the base of a PNP to case it to conduct) and NPN's are better at controlling voltages.

    So, how does one decide which devices to use (MOSFET vs BJT vs PNP vs NPN...)? Well, MOSFETS are extremely sensative to overvoltage/overcurrent. If a MOSFET data sheet says 10A--You don't exceed 10A's. If a BJT says 10A's well, a little bit over never hurt anyone. The benefits of a MOSFET are low resistance source to drain (MOSFETS consume almost no power while BJT's consume quite a bit of power). How do you decide when to use a P or N device? Well, as with the H-bridge I mentioned above you have to look at the application. PNP's are used to control current flow because they are not limited by needing a higher voltage on the base(or gate). NPN's(or N-channel FETS) are better amps because they require less power and are better suited to control voltages. Explaining why this is would require a little bit of xstor theory though and I'm too lazy to go through it all.

    Well, hope this helped. Google is your friend if your really interested in transistor theory.
  5. Apr 7, 2005 #4


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    I think I can describe the difference with minimal words.

    Voltage source: A source with a fixed voltage output that is able to deliver up to a specific current. A car battery is a voltage source.

    Current source: A source with a fixed current (amperes) output that is able to deliver that current within a certain range of voltage. For instance if you hook a 100 ohm resistor on a 1 amp current source the voltage out of the source will be 100 volts. If you hook a 10 ohm resistor onto the current source the voltage will fall to 10 volts. The current is the constant.
  6. Apr 8, 2005 #5
    Another question

    Just a question (which may be a bit stupid but you know what my lecturers say: the only stupid questions are those that are not asked :approve:).

    Looking at the config of an op amp, its, like full of transistors so it is a current source right? :blushing:
  7. Apr 8, 2005 #6
    Usually a op amp is used for as a controlled voltage source rather then a controlled current source.
  8. Apr 8, 2005 #7


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    A collection of building-blocks does not necessarily look like a building-block. That's the beauty of complexity. Op-amps can be used in a variety of ways -- VCVS, VCCS, CCVS, CCCS.

    - Warren
  9. Apr 8, 2005 #8
    Nope, though you could use one to make a current souce. Here are some comments... When doing circuit analysis, one represents a real-world current source as an ideal one in parallel with a conductance, and a real voltage source is shown as an ideal source in series with a resistance. An ideal current source would maintain a constant current even if it were open-circuited, which would create an infinite voltage. Likewise, an ideal voltage source would produce the same voltage if it were shorted, resulting in an infinite current. These scenarios can't exist in the real world. Also, you can convert a current source in || with a conductance into a V source in series with a resistance, and vice-versa. Read up on "Norton's Theorem" and "Thevenin's Theorem."
  10. Apr 8, 2005 #9
    As for the first part, the B-C junction is always reverse-biased. Yes, it conducts a little, because no transistor is perfect. That's leakage current, and the lower, the better. As for the differences you state between NPN & PNP transistors, I don't follow.... They have the same characteristics, only use opposite polarity biasing. In terms of basic operation, all BJTs are current-controlled current sources, and all FETs are voltage-controlled current sources, though any of them can perform in other ways by proper circuit design. Maybe you're thinking of the fact that PNPs are easier to make than NPNs with germanium, and the (forward) junction voltage drop for Ge is ~0.3V, as opposed to ~0.6V for Si?
  11. Apr 9, 2005 #10
    Just curious:

    The DC power supply in most labs are voltage sources or current sources? I think it should be a voltage source (a practical voltage source modelled as an ideal voltage source in series with a source resistance of 50 ohms) because when I test the power source in the lab with a DC motor, I can change the voltage across quite easily, but the current through the motor is not that steady (It shoots up a bit due to initial transient effects, then reaches a steady current value as shown on the meter of the lab power source.)
  12. Apr 9, 2005 #11
    Please explain exactly why my statement troubles you--I wasn't even considering Ge/Si BTW. Explain how a P device and an N device can have the same characteristics--reference a complementary pair if you would(I believe doing this will show that P and N devices are different: IRF9510 and IRF510 are a common complementary pair). How does current flow from collector to emitter withing a BJT (what PN junctions must the current cross traveling from a power source to a ground source in a NPN transistor)?

    Basically, what I'm trying to say is that I disagree with your assessment that PNP and NPN devices are different only in regards to biasing--even neglecting the internal construction of each device. P-substances have higher intrinsic resistances(in general) which limits the current carrying capabilities of said devices. If you look at a complementary pair--look at all of the data--you'll see there are operational differences. Sure 2n2907A's and 2n2222A's can be made with similar hfe, Ib, Ic, Ie, etc figures (these are small xstors); however, not all of the device data is the same. Looking at larger devices one can begin to see the differences between P and N devices.

    Here: 2n2222 fairchild datasheet http://www.fairchildsemi.com/ds/PN/PN2222A.pdf

    2n2907 fairchild datasheet http://www.solarbotics.net/library/datasheets/2N2907.pdf
  13. Apr 9, 2005 #12


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    Most decent lab type power supplies have both modes; constant current and constant voltage. You set the 'max' current to a given level and as the current demand from the load increases the current approaches this level and when it reaches this level the supply switches from a constant voltage to constant current. The supply now limits the current to the preset level and the voltage moves around to regulate it. This function of a power supply is mostly over-current protection. Some power supplies have what is called foldback current limiting. When a foldback limited supply has its output short circuited the current allowed to flow is much less than full rating current. Where simple current limiting such as the lab supply I described will allow max current to flow when the output is short circuited. The reason for foldback limiting is that the power supply will dissipate much less power in its internal circuits when short circuited.
  14. Apr 9, 2005 #13
    That means you can set it to be a constant voltage source or current source depending on the application you want. That is cool. What about a normal dry cell battery, or even 12V lead acid battery, are they voltage sources or current sources?
  15. Apr 9, 2005 #14
    They are considered voltage sources though there is a limit to both the current they can produce
  16. Apr 10, 2005 #15
    Something I am still not sure:

    The lead acid car battery is a voltage source, that means it supplies a constant voltage within a certain range of current flowing through it. However, in the real world, I understand that the car accumulator, like most batteries, have a max. current limit they can supply, due to electrochemical limitations.

    So using the same analogy, a practical current source can supply a constant current, within a certain range of voltage across, with a max. voltage it can supply, due to the same electrochemical limitations. Am I correct?
    Please correct me if I am wrong. I m still learning. Thanks.
  17. Apr 10, 2005 #16


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    That sounds perfectly accurate to me.

    - Warren
  18. Apr 10, 2005 #17
    You say, "...PNP's are used to control current flow because they are not limited by needing a higher voltage on the base(or gate). NPN's(or N-channel FETS) are better amps because they require less power and are better suited to control voltages...."
    PNPs & NPNs are both used to control current, and are used in similar applications. Both are equally good as amplifiers: whether they are primarily current or voltage amps depends on the external circuit components. And if you measure the voltage across the forward-biased E-B junction of a of a PNP it will be the same as that of a similar NPN. In a complentary amplifier they MUST operate in a more or less identical fashion, otherwise the output signal would be distorted. And I'll also say that if you take a circuit diagram for an amp, which uses an NPN (say, a 2N3904) and build it with a similar PNP (2N3905), and connect the power supply (and any other polarized components) with opposite polarity, it will perform the same.
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