How do you make a 2D character jump at a fixed height no matter how much you press jump?

Currently if I quickly tap the jump button my character will only jump slightly off the floor and if I hold it for a bit long will reach a max height. If I want to make the character only jump to one height regardless of how I press the jump button what in my code would I have to modify?

Current player controller code:

public class PlayerController : MonoBehaviour {

	public float maxSpeed = 5.0f;
	public float jumpHeight = 1.0f;
	public float checkRadius = 0.2f;
	public Transform groundChecker;
	public LayerMask whatIsGround;
	private bool performJump = false;
	private bool grounded;
	private bool facingRight = true;
	private Animator animationController;

	// Use this for initialization
	void Start () {
	
		animationController = GetComponent<Animator>();

	}
	
	// Update is called once per frame
	void Update () {

		//Checks whether the player wants to jump
		if (Input.GetAxis("Jump") > 0 && grounded){
			performJump = true;
		}
	
	}

	void FixedUpdate () {

		//Gets user input for moving along the horizontal axis
		float move = Input.GetAxis ("Horizontal");

		//Checks whether the player is on the ground
		grounded = Physics2D.OverlapCircle (groundChecker.position, checkRadius, whatIsGround);

		if (!grounded) {
			animationController.SetBool ("Grounded",false);
		}
		else{
			animationController.SetBool ("Grounded",true);
		}

		animationController.SetFloat ("Speed", Mathf.Abs(move));
		rigidbody2D.velocity = new Vector2 (maxSpeed * move, rigidbody2D.velocity.y);

		//Flips direction if moving in opposite direction to where character is facing
		if (facingRight && move < 0) {
			flipCharacter();
		}
		else if (!facingRight && move > 0){
			flipCharacter();
		}

		//Jumps if the jump boolean has been set to true
		if (performJump) {
			jump ();
			performJump = false;
		}

	}

	void flipCharacter(){
		//Flips the character based on local scale
		facingRight = !facingRight;
		Vector3 theScale = transform.localScale;
		theScale.x = theScale.x * -1;
		transform.localScale = theScale;
	}
	
	void jump(){
		rigidbody2D.AddForce (Vector2.up * jumpHeight,ForceMode2D.Impulse);
	}
}

I found out how to fix it, it was because I was using Input.GetAxis, rather than Input.GetButtonDown, so the force was applied multiple times.