Designing op amp oscillator with LM741

Op amp oscillators are widely used in electronic circuits for generating signals with a specific frequency and waveform. The LM741 is a popular and versatile operational amplifier that can be used for designing oscillator circuits. In this post, we will discuss LM741 oscillator circuit design, LM741 oscillator circuit simulation, and LM741 Colpitts op amp oscillator in detail.

LM741 Oscillator Circuit Design

The 741 op amp is a single operational amplifier that can be used for designing different types of oscillator circuits such as RC, LC, and crystal oscillators. The basic LM741 oscillator circuit consists of a feedback loop, which includes a frequency-determining network and an inverting amplifier.

To design an LM741 oscillator circuit, you need to select a suitable frequency-determining network, such as an RC network or an LC network, and a suitable amplifier configuration, such as an inverting amplifier or a non-inverting amplifier. The frequency of oscillation depends on the values of the frequency-determining components, and the gain of the amplifier determines the stability and amplitude of the oscillator waveform.

LM741 Colpitts Oscillator

The LM741 Colpitts oscillator design is a popular oscillator circuit that uses a capacitive voltage divider and an inductor to determine the oscillation frequency. The Colpitts oscillator is a type of LC oscillator, which is commonly used in radio frequency circuits.

To design an LM741 Colpitts oscillator, you need to select suitable values of capacitors and inductors to determine the frequency of oscillation. The LM741 operational amplifier is used as an inverting amplifier in the feedback loop, which provides the necessary gain and phase shift for oscillation.

Following shows the circuit diagram of LM741 based Colpitts Oscillator.

LM741 based Colpitts Oscillator Circuit diagram
As can be observed the LM741 op-amp is configured in inverting configuration. The feedback network consist of C1, C2 and L1. The output of the op-amp amplifier is connected to this LC feedback network and the output of the feedback network is feed into the inverting terminal of the op-amp. The resistors R4 and R1 are the inverting op-amp voltage gain setting resistors and the resistors R2,R3 and C4 are used to bias the non-inverting terminal at the mid-point of the +5V dc power supply, that is the potential at the non-inverting terminal is 2.5V. Therefore the output swings with 2.5V at its center. This op-amp power supply configuration is called single supply op-amp configuration. For this see how to operate LM741 with single supply.

The op-amp oscillator design starts with calculating the feedback network capacitors and inductor values for desired frequency of oscillation. Let says that we want to design an op-amp oscillator with frequency of 100KHz. The equation of frequency for Colpitts oscillator is,

\(f_o = \frac{1}{2 \pi \sqrt{L_1C_T}}\)   ---------->(1)

where, \(C_T = \frac{C_1C_2}{C_1+C_2} \)   ------>(2)

We can solve for the C1,C2 and L1 values by either selecting values for C1, C2 and then solve for L or we can either choose C1 or C2 and choose L and calculate either C1 or C2.

Whichever way we choose, the capacitor ratio must adhere to the Barkhausen criteria of oscillation. It states that, if \(A_v\) and \(\beta\) are the voltage gain of the amplifier and feedback fraction then for sustained oscillation around the closed loop we must have,

\(A_v \beta \ge 1\)   --------->(3)


\(\beta = \frac{C_1}{C_2}\)    ----->(4)

and, \(A_v = -\frac{R_4}{R_1}\)   ------>(5)

For \(\beta\) to be faction C1 must be smaller than C2.

Design Steps for Op-Amp oscillator

So the op-amp oscillator design involves the following steps:

a. selecting C1 and C2 values whose ratio should be less than 1 and calculating \(\beta\)

Let C1=10nF and C2=100nF then we have, \(\beta = \frac{C_1}{C_2}=0.1\)

b. calculate the value of L1 using the desired frequency and C1,C2 values

\(L_1 = \frac{1}{(2\pi f)^2 C_T}\)

Here, \(C_T = \frac{C_1C_2}{C_1+C_2} = \frac{10nF \times 100nF}{10nF + 100nF}=9.09nF\)

Hence, \(L_1 = \frac{1}{(2\pi 100KHz)^2 9.09nF}=278.63\mu H\)

This can also be computed using the  online oscillator calculator.

c. calculating voltage gain \(A_v\) from \(\beta\)

Since \(A_v \beta \ge 1\)   and \(\beta = 0.1\)  we have \(A_v \ge 10\)

d. choose R1 and then calculating R4 from values of \(A_v\) and R1

Let R1=1KOhm then we have,

\(R_4 = A_v \times R_1 = 10 \times 1K\Omega = 10k\Omega\)

LM741 Oscillator Circuit Simulation

Before implementing an LM741 oscillator circuit, it is essential to simulate the circuit using a software tool such as LTSpice or Proteus. Simulation helps to verify the design parameters and identify any potential issues or instability in the circuit.

To simulate an LM741 oscillator circuit, you need to build a schematic diagram of the circuit in the simulation software and specify the component values and operating conditions. You can then run the simulation and analyze the output waveform, frequency, and stability of the oscillator circuit.

 LM741 based Colpitts Oscillator Circuit simulation

In conclusion, the LM741 is a versatile operational amplifier that can be used for designing various types of oscillator circuits. By carefully selecting the frequency-determining components and amplifier configuration, you can design an LM741 oscillator circuit that meets your specific requirements. Simulating the circuit before implementation is essential for verifying the design parameters and identifying any potential issues or instability in the circuit. The LM741 Colpitts oscillator is a popular oscillator circuit that can be used in radio frequency circuits for generating stable and accurate signals. 


[1] Colpitts Oscillator in Matlab Simscape

[2] 2N5550 transistor Colpitts Oscillator on breadboard

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