Designing a BJT Switch for Vce Saturation

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Discussion Overview

The discussion revolves around the design of a BJT switch, specifically focusing on ensuring that the transistor is in Vce saturation. Participants explore various factors influencing saturation, including load conditions and base current requirements.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest that ensuring Vce saturation depends on the load at the collector and recommend driving more base current.
  • One participant specifies a scenario with a 100 ohm resistor load and a Vcc of 5V, using a 2N2222 BJT.
  • There are inquiries about the typical Vcesat for the 2N2222, the voltage drop across the resistor, and the current Ic required.
  • Participants discuss the need for a minimum Beta (β) at the specified Ic and operating temperature to determine the necessary base current for saturation.
  • One participant emphasizes that driving with more current is not just an easy solution but a necessary approach, noting that β drops significantly when saturated, requiring more base current.
  • Another participant highlights the variability of β, stating that it can change based on batch, device, current, and temperature conditions.

Areas of Agreement / Disagreement

Participants express differing views on the approach to achieving saturation, with some advocating for increased base current while others challenge this as a simplistic solution. The discussion remains unresolved regarding the best practices for ensuring saturation in various conditions.

Contextual Notes

Participants mention the importance of consulting the datasheet for the 2N2222 to understand the recommended base current for achieving saturation under specific conditions.

likephysics
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How do you design a BJT switch to ensure it is (Vce) saturated.
 
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It all depend what is the load at the collector, in general, drive more base current.
 
yungman said:
It all depend what is the load at the collector, in general, drive more base current.
Drive with more current is the easy way out.
Let's say the load is a 100 ohm resistor. Vcc is 5V.
BJT is 2N2222.
 
likephysics said:
Drive with more current is the easy way out.
Let's say the load is a 100 ohm resistor. Vcc is 5V.
BJT is 2N2222.

What is the typical Vcesat for the 2222? So then what will be the voltage drop across the resistor Rc? So then what will be the current Ic? What is the minimum Beta for the 2222 at that Ic and your operating temperature? So what base current is needed to ensure saturation?

See how it's done? Show us your work...
 
likephysics said:
Drive with more current is the easy way out.
Let's say the load is a 100 ohm resistor. Vcc is 5V.
BJT is 2N2222.

It is not the easy way out, it is the way. When saturated, β drop way low, you need way more base current. You ask how to make sure it is saturated!

5V 100Ω is 50mA. You want do drive at least 5mA to get good saturation. Read the data sheet, they drive 15mA for 150mA.
 
likephysics said:
Drive with more current is the easy way out.
Let's say the load is a 100 ohm resistor. Vcc is 5V.
BJT is 2N2222.
To ensure it is saturated you drive it with sufficient base current that under all foreseeable conditions, your base current IB > IC / β

Remember, β is far from a constant, it varies from batch to batch, from device to device, it varies with current, and it varies with temperature.

http://img803.imageshack.us/img803/4666/holly1756.gif
 
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