I think flalign* and \intertext{} are very good for you.

```
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{flalign*}
& & T_{Loop Filter}(s)&=K_{p}+\frac{K_{i}}{s} & & \\
\intertext{The overall transfer function of given PLL can be given as}
& & \frac{{\phi}_{c}(s)}{{\phi}(s)}&=\frac{T_{Loop Filter}(s)*\frac{1}{s}}{1+\frac{T_{Loop Filter}}(s)*\frac{1}{s}} & & \\
& & \frac{\phi_{c}(s)}{\phi(s)}&=\frac{K_{p}(s)+K_{i}}{s^{2}+K_{p}(s)+K_{i}} & &
\intertext{In general transfer function for a second order closed loop control system is given as}
& & T_{second order}&=\frac{{\omega}^2+{2{\delta}}{\omega}s}{s_{2}+2{\delta}{\omega}s+{\omega}^{2}} & & \\
& \rlap{\text{On comparing}} & {\omega}&={sqrt{K_i}} {\delta}=\frac{K_p}{2\sqrt{K_{i}}} & &
\end{flalign*}
\end{document}
```

This is a case where visual formatting seems necessary, so it should be dealt with only in the very last stage of document preparation.

You can pretend that the very last part of the formula “is not there”:

```
\documentclass[12pt, oneside]{scrbook}
\usepackage{amsmath,mathtools}
\DeclareMathOperator{\E}{E}
\newcommand{\with}{\,;}
\newcommand{\dx}{\mathop{}\!\mathrm{d}}
\newcommand{\expect}[2][]{\E\if\relax\detokenize{#1}\relax\else_{#1}\fi \left[#2\right]}
\begin{document}
\begin{equation}
\begin{aligned}
\mu_{D_-}
&= \expect{f'(S_-) \cdot \sum_{i=1}^{N} W D} \\
&= N \mu_W \mu_D \int_{-\infty}^{\infty} f'(s) p_\mathcal{N}(s \with \mu_{S_-}, \sigma_{S_-}) \dx{s} \\
\sigma_{D_-}^2
&= \expect{f'(S_-)^2 \cdot \left(\sum_{i=1}^{N} W D\right)^2} - \mu_{D_-}^2 \\
&= N(\sigma_W^2 \sigma_D^2 + \mu_W^2 \sigma_D^2 + \sigma_W^2 \mu_D^2 + N \mu_W^2 \mu_D^2)
\int_{-\infty}^{\infty} f'(s)^2 p_\mathcal{N}(s \with \mu_{S_-}, \sigma_{S_-}) \dx{s}
\mathrlap{{} - \mu_{D_-}^2 \,.}
\end{aligned}
\end{equation}
\end{document}
```

I amended your definition of `\dx`

, because it would give wrong spacing on the right. Also `\;`

seems too big for `\with`

(to be honest, I wouldn't add a space before the semicolon).

Your definition of `\expect`

with automatic `\left`

and `\right`

produces too big delimiters in many cases (see the third line, for instance).

Here's an amended version that uses `xparse`

and `mathtools`

features; the syntax for `\expect`

is

```
\expect{formula}
\expect[\big]{formula}
\expect*{formula}
\expect_{sub}{formula}
\expect_{sub}[\big]{formula}
\expect_{sub}*{formula}
```

as shown in the added equation. In the optional argument one can use `\big`

, `\Big`

, `\bigg`

or `\Bigg`

; the *-version uses `\left`

and `\right`

.

Note also the addition of `\,`

and `\!`

in a few places.

## Best Answer

Use the

`amsmath`

package and the commands`\text{...}`

for text in the formula or`\intertext{...}`

for text between the lines of multi-line formulas. For example:One advantage of

`align*`

to`\[ ... \]`

is that you can align the equations on relation symbols.If you wish to put

orin the same line, you could use`\text`

and`flalign*`

: