JK Flip Flop Truth Table and Circuit
Counter Circuits Electronics tutorial Flip Flops Logic Gates

JK Flip Flop Truth Table, Circuit Diagram, Working & Applications

Let’s explore JK Flip Flop Truth Table, with its circuit diagram. A flip-flop is a fundamental building block of digital circuits used in digital electronics and sequential logic circuits. It is a type of bistable multivibrator, meaning it has two stable states. Flip-flops are widely used for data storage, data transfer, and control applications in digital systems.

SR flip-flop is the simplest one having two inputs, S (set) and R (reset), and two outputs, Q and Q’. It changes state based on the inputs provided. However, it has a drawback known as the “invalid state” when both inputs are set to 1 simultaneously.

JK Flip-Flop is an enhancement of the SR flip-flop. It has two inputs, J (set) and K (reset), and two outputs, Q and Q’. It eliminates the “invalid state” problem of the SR flip-flop by adding feedback from the outputs.

Logic symbol of the JK flip-flop

JK Flip Flop Symbol
JK Flip Flop Symbol

The JK flip-flop has some interesting features compared to other flip-flops like the SR (Set-Reset) flip-flop. JK flip-flop is the most widely used universal flip-flop and it can be used in many ways. Here’s the truth table, circuit diagram, and how it works:

Truth Table of JK Flip Flop:

J K Q(t) Q(t+1)
0 0 Q(t) Q(t)
0 1 Q(t) 0
1 0 Q(t) 1
1 1 Q(t) ~Q(t)


  • Q(t) is the current state of the output.
  • Q(t+1) is the next state of the output.

Circuit Diagram of JK Flip Flop:

JK Flip Flop Circuit Diagram
JK Flip Flop Circuit Diagram

Three input Nand gates have been used in designing of JK flip flop. Nand gate is a universal logic gate.

Working of JK Flip Flop:

  • When J=0 and K=0: The output (Q) remains the same as its previous state, maintaining the current state (No change).
  • When J=0 and K=1: The output (Q) becomes 0, regardless of its previous state (Reset).
  • When J=1 and K=0: The output (Q) becomes 1, regardless of its previous state (Set).
  • When J=1 and K=1: The output (Q) toggles to its complement (Complement).

The key feature of the JK flip-flop is its ability to toggle when both J and K are set to 1. This is not present in other types of flip-flops like the SR flip-flop. This makes the JK flip-flop very versatile and useful in various applications including counters, shift registers, and memory storage circuits.

JK Flip Flop Truth Table:

JK Flip Flop Truth Table
JK Flip Flop Truth Table

Race Around Scenario in JK Flip-Flop:

The race-around condition happens when both J and K inputs are set to 1 (J = K = 1), and the clock pulse (CLK) remains high for an extended period. In this scenario, the output (Q) starts to toggle continuously between 0 and 1.

Why it Occurs:

  1. Feedback Loop: JK flip-flops have a feedback path where the output (Q) is connected back to one of the inputs.
  2. Propagation Delay: Every logic gate has a propagation delay, which is the time it takes for the output to change after an input change.

When J = K = 1, the flip-flop acts like a toggle switch. Here’s how the race unfolds:

  • Initially, let’s say Q = 0.
  • Due to the high clock pulse (CLK) and J = K = 1, the output (Q) changes to 1 after its propagation delay (Δt).
  • But Q’s new value (1) feeds back to the input due to the feedback loop.
  • After another propagation delay (Δt), the input change (Q becoming 1) propagates through the flip-flop, causing the output (Q) to switch back to 0.

This continuous back-and-forth change between 0 and 1 due to propagation delays creates an unstable and unpredictable output, hence the term “race-around condition.”

How to Avoid Race-Around Condition:

There are two main ways to eliminate the race-around condition:

  1. Master-Slave JK Flip-Flop: This is a modified version of the JK flip-flop that uses two cascaded JK flip-flops. One acts as a “master” stage and the other acts as a “slave” stage. It eliminates the timing problems associated with a standard JK flip-flop.

  2. Shorter Clock Pulses: Ideally, the clock pulse duration (T) should be significantly shorter than the propagation delay (Δt) of the flip-flop. This minimizes the window for the output change to propagate back and influence the next toggle cycle.

Master-Slave JK Flip-Flop

Standard JK flip-flops can suffer from a phenomenon known as race-around condition. This occurs when the J and K inputs are both set to 1 for an extended period. In this case, the output of the flip-flop oscillates between 0 and 1, making its behavior unpredictable.

Master-Slave JK flip-flops solve this problem by ensuring that the J and K inputs of the flip-flop are only ever seen by the circuitry when the clock signal is high. This prevents the race-around condition from occurring.

Master-Slave JK Flip Flop Circuit Diagram
Master-Slave JK Flip Flop Circuit Diagram

How does a Master-Slave JK Flip-Flop work?

A Master-Slave JK flip-flop consists of two JK flip-flops connected in series. The first flip-flop is the master, and the second flip-flop is the slave. The clock signal is applied to the master flip-flop, and the output of the master flip-flop is fed to the inputs of the slave flip-flop.

  • When the clock signal is high, the master flip-flop is active, and the slave flip-flop is inactive. The inputs J and K are sampled by the master flip-flop, and the output of the master flip-flop changes according to the truth table for a JK flip-flop.
  • When the clock signal goes low, the master flip-flop becomes inactive, and the slave flip-flop becomes active. The output of the master flip-flop is captured by the slave flip-flop, and the output of the slave flip-flop becomes the new output of the Master-Slave JK flip-flop.

Benefits of Master-Slave JK Flip-Flop

  • Eliminates race-around condition.
  • Makes the circuit more reliable.
  • Can be used in higher-speed applications.

Drawbacks of Master-Slave JK Flip-Flop

  • Requires more complex circuitry than a standard JK flip-flop.
  • Introduces a slight delay in the output of the flip-flop.

Applications of JK Flip-Flop

JK flip-flops, a type of sequential logic circuit, find applications in various digital electronic systems where memory or sequential behavior is required. Here are some common applications of JK flip-flops:


JK flip-flops are widely used in designing various types of counters such as binary counters, ripple counters, and synchronous counters. They can be used to count clock pulses or events in digital systems.


JK flip-flops are used in the construction of registers, which are digital circuits used to store data temporarily. Shift registers, parallel-in-serial-out (PISO) registers, and serial-in-parallel-out (SIPO) registers often utilize JK flip-flops.

Frequency Dividers:

JK flip-flops can be used to design frequency dividers, which are circuits that divide the input frequency by a fixed integer value. This is useful in clock generation circuits and in digital frequency synthesizers.

Memory Elements:

In digital systems, JK flip-flops are employed as memory elements to store binary information. They are used in RAM (Random Access Memory) cells, shift registers, and other memory structures.

State Machines:

JK flip-flops are key components in the implementation of finite-state machines (FSMs). FSMs are widely used in control logic, digital signal processing, and communication systems for tasks such as sequence recognition, protocol handling, and control sequencing.

Data Synchronization:

JK flip-flops can be used for data synchronization purposes in digital systems. For example, they can synchronize data from asynchronous sources with a clock signal.

Toggle Flip-Flop:

When both J and K inputs are tied together and fed with a clock signal, a JK flip-flop behaves as a toggle flip-flop. This functionality is used in applications where a signal needs to alternate its state with every clock pulse.

Pulse Generators:

By appropriately configuring the inputs of a JK flip-flop, it can function as a pulse generator, producing output pulses of specific durations or frequencies.

Frequency Synthesizers:

JK flip-flops are used in frequency synthesizers to generate output frequencies that are multiples of a reference frequency.

Digital Logic Circuits:

JK flip-flops are fundamental building blocks in the design of complex digital logic circuits such as microprocessors, microcontrollers, and application-specific integrated circuits (ASICs).

These are just a few examples of the diverse applications of JK flip-flops in digital electronic systems. Their versatility and usefulness make them essential components in modern digital circuit design.

What is Flip Flop Circuit Truth Table and Various Types of Flip Flops


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