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Transistor gain, what if there is not enough current?

What is “pull the collector below ground” and saturationBJT at saturationTransistor not supply enough currentIs the current gain value constant for a single BJT ?decreasing base current on saturated npn transistorCurrent flow in a BJT transistorTransistor power dissipation, current, and voltageconfused about pnp transistor current flowWhat are the real world limitations of using a transistor as a switch?Does a transistor reduce current flow?

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$begingroup$

I understand that a transistor will amplify current by a certain gain factor if it is below saturation.

But what if there is a resistor that reduces current?

So let's say there is 1mA between base and emitter and a gain of 100x that means there will be 100mA between collector and emitter?

But what if there is a 5v supply voltage and a 1000ohm resistor between supply and collector, it will be impossible for this much current to flow.

transistors

share|improve this question

asked 2 hours ago

user221241user221241

61

New contributor

user221241 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  That mode of operation is : saturation.
  $endgroup$
  – analogsystemsrf
  1 hour ago
  

  
  
  
  

add a comment | 

1

$begingroup$

I understand that a transistor will amplify current by a certain gain factor if it is below saturation.

But what if there is a resistor that reduces current?

So let's say there is 1mA between base and emitter and a gain of 100x that means there will be 100mA between collector and emitter?

But what if there is a 5v supply voltage and a 1000ohm resistor between supply and collector, it will be impossible for this much current to flow.

transistors

share|improve this question

asked 2 hours ago

user221241user221241

61

New contributor

user221241 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  That mode of operation is : saturation.
  $endgroup$
  – analogsystemsrf
  1 hour ago
  

  
  
  
  

add a comment | 

$begingroup$

I understand that a transistor will amplify current by a certain gain factor if it is below saturation.

But what if there is a resistor that reduces current?

So let's say there is 1mA between base and emitter and a gain of 100x that means there will be 100mA between collector and emitter?

But what if there is a 5v supply voltage and a 1000ohm resistor between supply and collector, it will be impossible for this much current to flow.

transistors

share|improve this question

asked 2 hours ago

user221241user221241

61

New contributor

user221241 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

share|improve this question

asked 2 hours ago

user221241user221241

61

New contributor

user221241 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

asked 2 hours ago

user221241user221241

61

New contributor

user221241 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 2 hours ago

user221241user221241

61

New contributor

user221241 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 2 hours ago

user221241user221241

61

2 Answers
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2

$begingroup$

I understand that a transistor will amplify current by a certain gain factor if it is below saturation.

Good.

But what if there is a resistor that reduces current?

Then the transistor will drive into saturation.

There are many times when this is useful - most notably in the NPN low-side switch. Here we want the transistor to behave like a switch so we inject a base current high enough to ensure that the transistor saturates so that the collector-emitter voltage, V_CE is as low as possible. This eliminates variation in load current due to variations in the transistor gain and also ensures that power dissipation in the transistor is at a minimum.

Figure 1. A typical NPN low-side switch can be driven into saturation by making R1 low enough. Source: LEDnique by the author.

To ensure saturation it is normal to assume a much lower current gain than the h_fe paramater might suggest. 10 to 20 is typical. See the linked article for a worked calculation.

share|improve this answer

answered 1 hour ago

TransistorTransistor

90.8k788195

$endgroup$

add a comment | 

1

$begingroup$

The purpose of biasing a transistor amplifier is to make sure that this problem does not occur. The resistor values are selected so that without any input signal (other than the dc bias) the output voltage will be about half way between ground and Vcc. That setup allows for the largest possible linear amplification.

share|improve this answer

answered 2 hours ago

Elliot AldersonElliot Alderson

8,34321122

$endgroup$

add a comment | 

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2

$begingroup$

I understand that a transistor will amplify current by a certain gain factor if it is below saturation.

Good.

But what if there is a resistor that reduces current?

Then the transistor will drive into saturation.

There are many times when this is useful - most notably in the NPN low-side switch. Here we want the transistor to behave like a switch so we inject a base current high enough to ensure that the transistor saturates so that the collector-emitter voltage, V_CE is as low as possible. This eliminates variation in load current due to variations in the transistor gain and also ensures that power dissipation in the transistor is at a minimum.

Figure 1. A typical NPN low-side switch can be driven into saturation by making R1 low enough. Source: LEDnique by the author.

To ensure saturation it is normal to assume a much lower current gain than the h_fe paramater might suggest. 10 to 20 is typical. See the linked article for a worked calculation.

share|improve this answer

answered 1 hour ago

TransistorTransistor

90.8k788195

$endgroup$

add a comment | 

2

$begingroup$

I understand that a transistor will amplify current by a certain gain factor if it is below saturation.

Good.

But what if there is a resistor that reduces current?

Then the transistor will drive into saturation.

There are many times when this is useful - most notably in the NPN low-side switch. Here we want the transistor to behave like a switch so we inject a base current high enough to ensure that the transistor saturates so that the collector-emitter voltage, V_CE is as low as possible. This eliminates variation in load current due to variations in the transistor gain and also ensures that power dissipation in the transistor is at a minimum.

Figure 1. A typical NPN low-side switch can be driven into saturation by making R1 low enough. Source: LEDnique by the author.

To ensure saturation it is normal to assume a much lower current gain than the h_fe paramater might suggest. 10 to 20 is typical. See the linked article for a worked calculation.

share|improve this answer

answered 1 hour ago

TransistorTransistor

90.8k788195

$endgroup$

add a comment | 

$begingroup$

I understand that a transistor will amplify current by a certain gain factor if it is below saturation.

Good.

But what if there is a resistor that reduces current?

Then the transistor will drive into saturation.

There are many times when this is useful - most notably in the NPN low-side switch. Here we want the transistor to behave like a switch so we inject a base current high enough to ensure that the transistor saturates so that the collector-emitter voltage, V_CE is as low as possible. This eliminates variation in load current due to variations in the transistor gain and also ensures that power dissipation in the transistor is at a minimum.

Figure 1. A typical NPN low-side switch can be driven into saturation by making R1 low enough. Source: LEDnique by the author.

To ensure saturation it is normal to assume a much lower current gain than the h_fe paramater might suggest. 10 to 20 is typical. See the linked article for a worked calculation.

share|improve this answer

answered 1 hour ago

TransistorTransistor

90.8k788195

$endgroup$

share|improve this answer

answered 1 hour ago

TransistorTransistor

90.8k788195

answered 1 hour ago

TransistorTransistor

90.8k788195

answered 1 hour ago

TransistorTransistor

90.8k788195

1

$begingroup$

The purpose of biasing a transistor amplifier is to make sure that this problem does not occur. The resistor values are selected so that without any input signal (other than the dc bias) the output voltage will be about half way between ground and Vcc. That setup allows for the largest possible linear amplification.

share|improve this answer

answered 2 hours ago

Elliot AldersonElliot Alderson

8,34321122

$endgroup$

add a comment | 

1

$begingroup$

The purpose of biasing a transistor amplifier is to make sure that this problem does not occur. The resistor values are selected so that without any input signal (other than the dc bias) the output voltage will be about half way between ground and Vcc. That setup allows for the largest possible linear amplification.

share|improve this answer

answered 2 hours ago

Elliot AldersonElliot Alderson

8,34321122

$endgroup$

add a comment | 

$begingroup$

The purpose of biasing a transistor amplifier is to make sure that this problem does not occur. The resistor values are selected so that without any input signal (other than the dc bias) the output voltage will be about half way between ground and Vcc. That setup allows for the largest possible linear amplification.

share|improve this answer

answered 2 hours ago

Elliot AldersonElliot Alderson

8,34321122

$endgroup$

share|improve this answer

answered 2 hours ago

Elliot AldersonElliot Alderson

8,34321122

answered 2 hours ago

Elliot AldersonElliot Alderson

8,34321122

answered 2 hours ago

Elliot AldersonElliot Alderson

8,34321122