Transistors - Field Effect and Bipolar Transistors: MOSFETS and BJTs

00:12:17
https://www.youtube.com/watch?v=Bine_PbyFSQ

摘要

TLDRThe video covers the principles of Bipolar Junction Transistors (BJTs) and Field Effect Transistors (FETs). It explains how BJTs use current to control a larger current through their terminals, while FETs use voltage. It also discusses the two types of BJTs (NPN and PNP) and their current flow directions, as well as the two types of FETs (N Channel and P Channel) and how they operate based on voltage differences. Additionally, it highlights the differences in their behaviors when used as switches and notes the presence of inherent components like diodes and capacitors in MOSFETs that affect their operation.

心得

  • 🔴 Red spheres symbolize positive current.
  • 🔄 Bipolar Junction Transistors control larger currents with a small input current.
  • ⚙️ Field Effect Transistors control currents with voltage at the middle terminal.
  • 🔄 NPN transistors have current flowing into the base, while PNP have current flowing out.
  • ⚡ Threshold voltage is crucial for FET operation.
  • 💡 Collector current in BJTs is proportional to the base current.
  • ⚙️ MOSFETs act like resistors when on, not ideal switches.
  • 🚫 MOSFETs can't block reverse current due to a built-in diode.
  • 💡 MOSFETs have inherent capacitors that affect gate current during voltage changes.
  • ⚡ Switching behaviors differ significantly between BJTs and MOSFETs.

时间轴

  • 00:00:00 - 00:12:17

    Further distinctions between N-channel and P-channel MOSFETs are explained, emphasizing the importance of gate-source voltage exceeding a threshold for current flow. It’s noted that, unlike Bipolar Transistors, the behavior of MOSFETs does not allow for significant current increase with rising drain voltage, but a decrease in drain voltage can significantly lower current. The operational characteristics of Bipolar Transistors are compared, with a constant voltage difference between base and emitter, reflecting a switch-like behavior, though not ideal. MOSFETs also demonstrate switch behavior but with inherent diode and capacitor functionalities affecting their performance.

思维导图

视频问答

  • What are Bipolar Junction Transistors?

    Bipolar Junction Transistors (BJTs) are transistors where a small current flowing into the middle terminal controls a larger current through the other terminals.

  • How do Field Effect Transistors differ from BJTs?

    Field Effect Transistors (FETs) use voltage at the middle terminal to control current, rather than a current flowing through it.

  • What are the types of BJTs?

    The two main types of BJTs are NPN and PNP transistors.

  • What are MOSFETs?

    MOSFETs are a type of Field Effect Transistor with two main types: N Channel and P Channel.

  • What controls the current in an N Channel MOSFET?

    In an N Channel MOSFET, the current is controlled by the voltage difference between the Gate and Source.

  • What is 'threshold voltage'?

    Threshold voltage is the minimum voltage required between the Gate and Source for the transistor to start conducting.

  • What is the voltage difference for BJTs when active?

    When a BJT is active, the difference between the base voltage and emitter voltage is about 0.7 volts.

  • How does a MOSFET behave as a switch?

    When a MOSFET is on, it behaves like a resistor with a small resistance value, not an ideal switch.

  • What current flows when the gate voltage changes in a MOSFET?

    A brief momentary current flows through the gate terminal when the gate voltage changes due to the charging and discharging of inherent capacitors.

  • Why can't MOSFETs block in the reverse direction?

    MOSFETs cannot block in the reverse direction because of an inherent diode built into their body.

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自动滚动:
  • 00:00:04
    The red spheres represent positive current,
  • 00:00:07
    which is in the opposite direction of electron current.
  • 00:00:16
    In a “Bipolar Junction Transistor”,
  • 00:00:19
    a small current flowing into the middle terminal
  • 00:00:23
    controls a much larger current through the transistor’s two other terminals.
  • 00:00:35
    Bipolar Transistors are one of the two main types of transistors.
  • 00:00:44
    In this other type of transistor,
  • 00:00:47
    the middle terminal does not have any current
  • 00:00:49
    passing through it during steady state conditions.
  • 00:01:00
    Here, it is the “voltage” of the middle terminal that controls
  • 00:01:03
    the current through the other two terminals.
  • 00:01:14
    This is called a “Field Effect Transistor.”
  • 00:01:23
    There are two main types of “Field Effect Transistors”
  • 00:01:27
    and two main types of “Bipolar Transistors.”
  • 00:01:36
    We can tell them apart by whether the arrow
  • 00:01:39
    is pointing toward or away from the middle terminal.
  • 00:01:49
    In bipolar transistors, the positive current flows
  • 00:01:53
    in the direction of the arrow.
  • 00:02:01
    The red spheres symbolize the positive current.
  • 00:02:12
    This is a “NPN” bipolar transistor.
  • 00:02:29
    This is a “PNP” bipolar transistor.
  • 00:02:43
    Here, it is the positive current flowing “out”
  • 00:02:45
    of the middle terminal that controls the transistor.
  • 00:03:04
    Let’s now focus on Field Effect Transistors.
  • 00:03:09
    The two main types of “Field Effect transistors” are two different types of
  • 00:03:13
    “Metal Oxide Semiconductor Field Effect Transistors”, abbreviated “MOSFETs.”
  • 00:03:34
    This is an “N Channel” MOSFET.
  • 00:03:42
    As always, the red spheres represent the direction of positive current.
  • 00:03:51
    Here, a decrease in the voltage on the middle terminal
  • 00:03:54
    will cause a decrease in current.
  • 00:04:02
    An increase in the voltage on the middle terminal
  • 00:04:05
    will cause an increase in current.
  • 00:04:18
    This relationship is reversed for a “P Channel” MOSFET.
  • 00:04:44
    In both cases, the voltage that controls the transistor is the voltage
  • 00:04:49
    between the terminals that we call the “Gate” and the “Source.”
  • 00:05:02
    In the N channel example, the “Source” is shown at the bottom.
  • 00:05:19
    In the P channel example, the “Source” is shown at the top.
  • 00:05:37
    In both cases, the voltage difference between the “Gate” and the “Source”
  • 00:05:42
    has to exceed a certain “threshold voltage” before it starts having any effect.
  • 00:06:12
    Since it is the voltage between the “Gate” and the “Source” that controls the transistor,
  • 00:06:17
    the current will not increase much if we increase the voltage at the “Drain.”
  • 00:06:33
    But, if we reduce the “Drain” voltage,
  • 00:06:36
    we will eventually reach a point where the current will significantly decrease.
  • 00:07:22
    The behavior of Bipolar Transistors is different than Field Effect Transistors,
  • 00:07:28
    and the terminals have different names.
  • 00:07:47
    Here, when the current is flowing,
  • 00:07:50
    the difference between the “base” voltage and the “emitter” voltage
  • 00:07:53
    stays at about 0.7 volts,
  • 00:07:56
    and the “collector” current is the “base” current multiplied by a large constant number.
  • 00:08:27
    Suppose we add another resistor as shown.
  • 00:08:34
    A change in the current through this resistor
  • 00:08:37
    causes a change in the voltage drop across it.
  • 00:08:54
    Here, the collector voltage is 0.3 volts above the emitter voltage.
  • 00:09:01
    If the base current is increased further,
  • 00:09:03
    the collector voltage is unable to go any lower.
  • 00:09:43
    Here, the all the currents are staying constant.
  • 00:10:22
    Here, the bipolar transistor is behaving like a switch in the on position,
  • 00:10:28
    but it is not an ideal switch because there is a 0.3 volt drop across it.
  • 00:10:42
    On the other hand, when a “MOSFET” behaves like a switch in the on position,
  • 00:10:48
    it is not an ideal switch because it behaves like a resistor
  • 00:10:51
    with a small resistance value.
  • 00:11:04
    A MOSFET is unable to block in the reverse direction
  • 00:11:07
    because there is a diode inherently built into its body.
  • 00:11:24
    Also inherently built into each MOSFET
  • 00:11:27
    are capacitors between each of the terminals.
  • 00:11:36
    Although it is not shown in this video, these capacitors need to charge and discharge,
  • 00:11:43
    and there is therefore a brief momentary current through the gate terminal
  • 00:11:47
    when the gate voltage changes.
标签
  • Bipolar Junction Transistor
  • Field Effect Transistor
  • MOSFET
  • NPN
  • PNP
  • N Channel MOSFET
  • P Channel MOSFET
  • threshold voltage
  • current control
  • switching behavior