Mid-cap stocks are generally classified as companies with a market capitalization between $2 billion and $10 billion. They are ideal for those investors who don't have the risk appetite to buy small-cap stocks, but would like to enjoy stronger returns when compared to large-caps.

That's because mid-cap stocks tend to be less volatile than small-caps. They can also generate stronger returns because of their ability to quickly adapt to changes.

Let's take a closer look at two mid-cap stocks -- Synaptics (NASDAQ:SYNA) and Cirrus Logic (NASDAQ:CRUS) -- that have set the stock market on fire in recent months, and explore why they could make a nice addition to investors' portfolios.

Person pressing the buy button on a keyboard

IMAGE SOURCE: GETTY IMAGES.

Synaptics is making a comeback

Synaptics stock has more than doubled in the past six months -- graduating from small-cap to mid-cap in a short time -- as its turnaround is gathering momentum. The chipmaker, which is known for supplying human interface solutions such as touchscreen controllers for smartphones, was struggling earlier due to a downturn in the smartphone space.

But things are changing now in the mobile business that supplies 54% of Synaptics' total revenue. The chipmaker is witnessing strength thanks to the adoption of OLED displays, which is leading to a strong demand for its touchscreen controllers. Synaptics has already scored a big win in the form of Huawei, which is using the former's touchscreen controller to power its flagship smartphone.

Supply chain rumors now suggest that Synaptics is all set to supply its touchscreen controllers for this year's Apple (NASDAQ:AAPL) iPhones. John Vinh, an analyst at KeyBanc Capital Markets, upgraded Synaptics stock to "overweight," or buy with an $80 price target, recently citing a potential design win at Apple.

With Apple expected to build 100 million units of its next-gen iPhones in 2020, Synaptics could win big from this opportunity.

It is difficult to know how much dollar content Synaptics could land in each unit of the new iPhone this year. But even a couple of dollars' worth of business from each unit could give the chipmaker a double-digit boost as Synaptics' trailing 12-month revenue stands at just $1.39 billion.

That's probably the reason why analyst estimates compiled by Yahoo! Finance predict a 2% jump in Synaptics' top line in the next fiscal year that begins in July this year. For comparison, Synaptics' top line is expected to drop in the double digits in the ongoing fiscal year.

The company's bottom line is also expected to increase in the current and oncoming fiscal year, thanks to its transition to high-margin products in the mobile business. It won't be surprising to see Synaptics stock rise further this year as investors buy more of the stock to take advantage of the potential earnings growth, given its attractive forward earnings multiple.

The stock seems expensive at first given its trailing price-to-earnings (P/E) multiple of nearly 52. However, a forward P/E multiple of 18 indicates impressive bottom-line growth that could translate into more stock upside.

Of course, the risk of Synaptics not winning the Apple business is there. But given the company's history of being a supplier to Cupertino, and the fact that the overall flexible OLED display market is anticipated to clock 40% annual growth through 2025 -- according to Mordor Intelligence -- opens up a big opportunity for Synaptics to tap into.

Cirrus Logic's turnaround is already here

Cirrus Logic stock has nearly doubled in the past six months, and it now has a market cap of nearly $5 billion. Like Synaptics, the turnaround in Cirrus' financial fortunes is the reason why the stock has shot up impressively in recent months.

Cirrus is back to revenue growth as the fortunes of its largest customer -- Apple -- have turned around with the iPhone 11 this year. With more than 80% of its revenue coming from supplying audio chips to Apple, Cirrus Logic is a great way to play the potential jump in iPhone sales in 2020 and beyond.

After all, Apple is expected to launch a range of iPhone devices this year at various price points. Noted Apple analyst Ming-Chi Kuo recently wrote in a research note than Cupertino could launch as many as five iPhone models in 2020. The cheapest model could be the next iteration of the budget-friendly iPhone SE that's meant to help Apple cut its teeth in price-sensitive markets such as India.

Also, as Apple is expected to equip its 2020 iPhones with 5G (fifth-generation) wireless capabilities, their sales are expected to increase this year thanks to 5G deployments in China and the U.S.

In all, the iPhone maker's combination of affordable pricing and the inclusion of 5G are great news for Cirrus' business. The chipmaker's addressable opportunity is likely to increase as more customers upgrade to new iPhones, leading to a potential increase in iPhone builds this year.

Not surprisingly, Cirrus Logic's earnings are expected to jump to $3.39 per share in the current fiscal year from $2.64 in the previous one, according to consensus estimates. Estimates for the new fiscal year that begins in April are conservative at present, with analysts expecting 3.8% top-line growth and a slight bump in revenue.

But Cirrus can blow past those estimates later in the year when the production of this year's iPhone models ramps up. And with a forward P/E ratio of 28, it makes sense to buy Cirrus Logic stock, as it is capable of sustaining its status as a growth pick thanks to a potential outperformance in the cards.