# Simple circuit with a transistor

• dancavallaro
In summary: So the current through R_load does not depend on its value. In summary, the problem involves a voltage divider connected to a common-emitter amplifier circuit with a +10 V line, NPN transistor, and various resistors. The goal is to calculate the current through a specific resistor, R_load, and determine if it depends on the value of R_load. Using a simple model of the transistor, it is determined that the current through R_load is always 1 mA, regardless of the value of R_load.
dancavallaro

## Homework Statement

Here's a crude description of the problem until my attachment is approved: it's essentially a voltage divided connected to a common-emitter amplifier (I think). There's a +10 V line connected via an 8.4 kOhm resistor to the base of the NPN transistor, and a 1.6 kOhm resistor connecting the base to ground. Then there is a resistor R_load connecting the +10 V line to the collector of the transistor, and a 1.0 kOhm resistor connecting the emitter of the transistor to ground. I'm supposed to calculate the current through R_load, and say whether the result depends on the value of R_load. We're assuming the simplest model of a transistor: the base current is always 0, and when the B-E junction is forward-biased at least 0.6 V, then current flows from C to E, where V_E = V_C - 0.6.

## Homework Equations

When V_BE > 0.6:
I_C = I_E and V_E = V_C - 0.6

## The Attempt at a Solution

I think V_B = V_in*(1.6/(1.6+8.4)) = 1.6 V, so the B-E junction is forward-biased, and current flows from the collector to the emitter. Then V_C = 10 - I_C*R_load, but I_C = I_E so V_C = 10 - I_E*R_load. Then V_E = V_C - 0.6, so V_E = 9.4 - I_E*R_load. Then I_E = (9.4 - I_E*R_load)/1000, so I_E = 9.4/(1000+R_load). But I don't think this is right, because I'm pretty sure the current through R_load doesn't depend on the value of R_load. What am I doing wrong here?

#### Attachments

• Picture 1.png
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If the current through the 1kΩ resistor is greater than 100 ma doesn't the C-E open because the base is at 1.6v?

I'm not sure I follow.. why is R_load constrained to be greater than 9kΩ?

dancavallaro said:
I'm not sure I follow.. why is R_load constrained to be greater than 9kΩ?

Maybe I'm not understanding, I just thought that you said the voltage from the base to the emitter needed to be greater than .6v. Since the base is held at 1.6v I thought that meant the Ve couldn't be greater than 1v. So long as the Rload is greater than 9kΩ then the Ve will be <1v.

I shouldn't comment, having done nothing with transistors for many years, but it seems to me the absence of any specs on the transistor - such as its beta - means a crude answer is desired. I was taught that the emitter-base voltage on a working transistor is always 0.6 V. The voltage divider on the base puts the base at 1.6 V. So there must be 1 volt on the 1k emitter resistor. That determines the current regardless of the collector resistor!

Delphi51 said:
I shouldn't comment, having done nothing with transistors for many years, but it seems to me the absence of any specs on the transistor - such as its beta - means a crude answer is desired. I was taught that the emitter-base voltage on a working transistor is always 0.6 V. The voltage divider on the base puts the base at 1.6 V. So there must be 1 volt on the 1k emitter resistor. That determines the current regardless of the collector resistor!

That's right - we're using a very simple model right now, where beta is ignored and we assume that no current flows from base to emitter. What you say makes sense though - so if there's 1 volt on the 1k emitter resistor, then I_E = I_C = 1/1000 = 1 mA, regardless of the value of R_load.

## 1. How does a transistor work in a simple circuit?

A transistor is a semiconductor device that can amplify or switch electronic signals. In a simple circuit, the transistor acts as a switch by either allowing or blocking the flow of electricity between two terminals, known as the collector and emitter. The flow of electricity is controlled by a third terminal called the base, which acts as a gatekeeper for the current. When a small current is applied to the base, it allows a larger current to flow from the collector to the emitter, effectively amplifying the signal.

## 2. What are the main components of a simple circuit with a transistor?

A simple circuit with a transistor typically consists of three components: a power source, a transistor, and a load. The power source provides the energy for the circuit, the transistor acts as a switch or amplifier, and the load is the component that is powered by the circuit, such as a light bulb or motor.

## 3. How can a transistor be used to amplify a signal?

A transistor can be used to amplify a signal by using a small input current to control a larger output current. In a simple circuit, the input current is applied to the base terminal, which controls the flow of a larger current from the collector to the emitter. By adjusting the input current, the output current can be amplified to a desired level, making transistors useful in a variety of electronic devices, such as amplifiers and radios.

## 4. What are the advantages of using a transistor in a circuit?

There are several advantages to using a transistor in a circuit. Firstly, transistors are small in size and can be easily integrated into electronic devices, making them ideal for compact designs. They also have a low power consumption and can switch on and off quickly, making them efficient and reliable. Additionally, transistors can handle a wide range of voltages, making them versatile for use in different types of circuits.

## 5. What are some common applications for a simple circuit with a transistor?

Transistors are used in a variety of electronic devices and circuits, including amplifiers, radios, and computers. They are also used in power supplies, voltage regulators, and switching circuits. In addition, transistors are found in everyday household appliances, such as televisions, refrigerators, and washing machines. They are also essential components in modern technology, such as smartphones, laptops, and other portable devices.

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